There are actually several approaches to do this. Some require more overhead than others, and some are considered better than others.

In no particular order:

1. Use AJAX to get the data you need from the server.
2. Echo the data into the page somewhere, and use JavaScript to get the information from the DOM.
3. Echo the data directly to JavaScript.

In this post, we'll examine each of the above methods, and see the pros and cons of each, as well as how to implement them.

1. Use AJAX to get the data you need from the server

This method is considered the best, because your server side and client side scripts are completely separate.

Pros

• Better separation between layers - If tomorrow you stop using PHP, and want to move to a servlet, a REST API, or some other service, you don't have to change much of the JavaScript code.
• More readable - JavaScript is JavaScript, PHP is PHP. Without mixing the two, you get more readable code on both languages.
• Allows for async data transfer - Getting the information from PHP might be time/resources expensive. Sometimes you just don't want to wait for the information, load the page, and have the information reach whenever.
• Data is not directly found on the markup - This means that your markup is kept clean of any additional data, and only JavaScript sees it.

Cons

• Latency - AJAX creates an HTTP request, and HTTP requests are carried over network and have network latencies.
• State - Data fetched via a separate HTTP request won't include any information from the HTTP request that fetched the HTML document. You may need this information (e.g. if the HTML document is generated in response to a form submission) and, if you do, will have to transfer it across somehow. If you have ruled out embedding the data in the page (which you have if you are using this technique) then that limits you to cookies/sessions which may be subject to race conditions.

Implementation Example

With AJAX, you need two pages, one is where PHP generates the output, and the second is where JavaScript gets that output:

get-data.php

/* Do some operation here, like talk to the database, the file-session
 * The world beyond, limbo, the city of shimmers, and Canada.
 * 
 * AJAX generally uses strings, but you can output JSON, HTML and XML as well. 
 * It all depends on the Content-type header that you send with your AJAX
 * request. */

echo json_encode(42); //In the end, you need to echo the result. 
                      //All data should be json_encode()d.

                      //You can json_encode() any value in PHP, arrays, strings,
                      //even objects.

index.php (or whatever the actual page is named like)

<!-- snip -->
<script>
    function reqListener () {
      console.log(this.responseText);
    }

    var oReq = new XMLHttpRequest(); //New request object
    oReq.onload = function() {
        //This is where you handle what to do with the response.
        //The actual data is found on this.responseText
        alert(this.responseText); //Will alert: 42
    };
    oReq.open("get", "get-data.php", true);
    //                               ^ Don't block the rest of the execution.
    //                                 Don't wait until the request finishes to 
    //                                 continue.
    oReq.send();
</script>
<!-- snip -->

The above combination of the two files will alert 42 when the file finishes loading.

Some more reading material

• Using XMLHttpRequest - MDN
• XMLHttpRequest object reference - MDN
• How do I return the response from an asynchronous call?

2. Echo the data into the page somewhere, and use JavaScript to get the information from the DOM

This method is less preferable to AJAX, but it still has its advantages. It's still relatively separated between PHP and JavaScript in a sense that there is no PHP directly in the JavaScript.

Pros

• Fast - DOM operations are often quick, and you can store and access a lot of data relatively quickly.

Cons

• Potentially Unsemantic Markup - Usually, what happens is that you use some sort of <input type=hidden> to store the information, because it's easier to get the information out of inputNode.value, but doing so means that you have a meaningless element in your HTML. HTML has the <meta> element for data about the document, and HTML 5 introduces data-* attributes for data specifically for reading with JS that can be associated with particular elements.
• Dirties up the Source - Data that PHP generates is outputted directly to the HTML source, meaning that you get a bigger and less focused HTML source.
• Harder to get structured data - Structured data will have to be valid HTML, otherwise you'll have to escape and convert strings yourself.
• Tightly couples PHP to your data logic - Because PHP is used in presentation, you can't separate the two cleanly.

Implementation Example

With this, the idea is to create some sort of element which will not be displayed to the user, but is visible to JavaScript.

index.php

<!-- snip -->
<div id="dom-target" style="display: none;">
    <?php 
        $output = "42"; //Again, do some operation, get the output.
        echo htmlspecialchars($output); /* You have to escape because the result
                                           will not be valid HTML otherwise. */
    ?>
</div>
<script>
    var div = document.getElementById("dom-target");
    var myData = div.textContent;
</script>
<!-- snip -->

3. Echo the data directly to JavaScript

This is probably the easiest to understand, and the most horrible to use. Don't do this unless you know what you're doing.

Pros

• Very easily implemented - It takes very little to implement this, and understand.
• Does not dirty source - Variables are outputted directly to JavaScript, so the DOM is not affected.

Cons

• Insecure - PHP has no trivial JavaScript escape functions, and they aren't trivial to implement. Especially when using user inputs, you are extremely vulnerable to second tier injections. Disputed see comments
• Tightly couples PHP to your data logic - Because PHP is used in presentation, you can't separate the two cleanly.
• Structured data is hard - You can probably do JSON... kinda. But XML and HTML will require special attention.

Implementation Example

Implementation is relatively straightforward:

<!-- snip -->
<script>
    var data = <?php echo json_encode("42", JSON_HEX_TAG); ?>; //Don't forget the extra semicolon!
</script>
<!-- snip -->

Good luck!

I'm going to try a simpler answer:

Explanation of the problem

First, let's understand the flow of events when a page is served from our server:

• First PHP is run, it generates the HTML that is served to the client.
• Then, the HTML is delivered to the client, after PHP is done with it, I'd like to emphasize that once the code leaves the server - PHP is done with it and can no longer access it.
• Then, the HTML with JavaScript reaches the client, which can execute JS on that html.

So really, the core thing to remember here is that HTTP is stateless. Once a request left the server, the server can not touch it. So, that leaves our options to:

1. Send more requests from the client after the initial request is done.
2. Encode what the server had to say in the initial request.

Solutions

That's the core question you should be asking yourself is:

Am I writing a website or an application?

Websites are mainly page based, and the page load times needs to be as fast as possible (for example - Wikipedia) . Web applications are more AJAX heavy and perform a lot of round trips to get the client fast information (for example - a stock dashboard).

Website

Sending more requests from the client after the initial request is done is slow as it requires more HTTP requests which have significant overhead. Moreover, it requires asynchronousity as making an AJAX request requires a handler for when it's complete.

I would not recommend making another request unless your site is an application for getting that information from the server.

You want fast response times which have a huge impact on conversion and load times. Making ajax requests is slow for the initial uptime in this case and unneeded.

You have two ways to tackle the issue

• Set a cookie - cookies are headers sent in HTTP requests that both the server and client can read.
• Encode the variable as JSON - JSON looks very close to JavaScript objects and most JSON objects are valid JavaScript variables.

Setting a cookie is really not very difficult, you just assign it a value:

setcookie("MyCookie", $value); // sets the cookie to the value, remember, do not
                               // set it with HTTP only to true.

Then, you can read it with JavaScript using document.cookie:

Here is a short hand rolled parser, but the answer I linked to right above this has better tested ones:

var cookies = document.cookie.split(";").
    map(function(el){ return el.split("="); }).
    reduce(function(prev,cur){ prev[cur[0]] = cur[1];return prev },{});

cookies["MyCookie"] // value set with php.

Cookies are good for a little data. This is what tracking services often do.

Once we have more data, we can encode it with JSON inside a JS variable instead:

<script>
    var myServerData = <?=json_encode($value)?>; // don't forget to sanitize 
                                                 //server data
</script>

Assuming $value is json_encodeable on the PHP side (it usually is). This technique is what * does with its chat for example (only using .net instead of php).

Application

If you're writing an application - suddenly the initial load time isn't always as important as the ongoing performance of the app and it starts to pay off to load data and code separately.

My answer here explains how to load data using AJAX in JavaScript:

function callback(data){
    // what do I do with the response?
}

var httpRequest = new XMLHttpRequest;
httpRequest.onreadystatechange = function(){
    if (httpRequest.readyState === 4) {// request is done
        if (httpRequest.status === 200) {// successfully
            callback(httpRequest.responseText);// we're calling our method
        }
    }
};
httpRequest.open('GET', "/echo/json");
httpRequest.send();

Or with jQuery:

$.get("/your/url").done(function(data){
    // what do I do with the data?
});

Now, the server just needs to contain a /your/url route/file that contains code that grabs the data and does something with it, in your case:

<$php
 ...
 $val = myService->getValue(); // makes an api and db call
 echo json_encode($val); // write it to the output
 $>

This way, our JS file asks for the data and shows it rather than asking for code or for layout. This is cleaner and starts to pay off as the application gets higher. It's also better separation of concerns and it allows testing the client side code without any server side technology involved which is another plus.

Postscript: You have to be very aware of XSS attack vectors when you inject anything from PHP to JavaScript. It's very hard to escape values properly and it's context sensitive. If you're unsure how to deal with XSS, or unaware of it - please read this OWASP article, this one and this question.

myPlugin.start($val); // tried this, didn't work    

It doesn't work because $val is undefined as far as javascript is concerned, ie. the php did not output anything for $val. Try viewing the source in your browser and here is what you'll see:

myPlugin.start(); // tried this, didn't work    

And

<?php myPlugin.start($val); ?> // this didn't work either

This doesn't work because php will try to treat myPlugin as a constant and when that fails it will try to treat it as the string 'myPlugin' which it will try to concatenate with the output of the php function start() and since that is undefined it will produce a fatal error

And

 myPlugin.start(<?=$val?> // this works sometimes, but sometimes it fails

While this is most likely to work, since the php is producing valid javascript with the expected arguments, if it fails, chances are its because myPlugin isn't ready yet. Check your order of execution.

Also you should note that the php out put is insecure and should be filtered with json_encode()

EDIT

Because I didn't notice the missing parenthesis in myPlugin.start(<?=$val?> :-\

As @Second Rikudo points out, for it to work correctly $val would need to contain the closing parenthesis eg: $val="42);"

Meaning that the php will now produce myPlugin.start(42); and will work as expected when executed by the javascript

<script>
  var jsvar = <?php echo json_encode($PHPVar); ?>;
</script>

json_encode() requires:

• PHP 5.2.0 or more
• $PHPVar encoded as UTF-8, Unicode.

try this

<?php
    echo "<script> var x = ". json_encode($phpVariable)."</script>";
?>

--

-After trying this for a while Although it works , however it slows down the performance. as php is a server side script while javascript is a user side.

I quite like the way the Wordpress works with its enqueue and localize functions, so following that model, i wrote a simple class for putting a scripts into page according to the script dependencies, and for making additional data available for the script.

class mHeader {

    private $scripts = array();

    /**
     * @param string $id        unique script identifier
     * @param string $src   script src attribute
     * @param array  $deps      an array of dependencies ( script identifiers ).
     * @param array  $data      an array, data that will be json_encoded and available to the script.
     */
    function enqueue_script( $id, $src, $deps = array(), $data = array() ) {
        $this->scripts[$id] = array( 'src' => $src, 'deps' => $deps, 'data' => $data );
    }

    private function dependencies( $script ) {
        if ( $script['deps'] ) {
            return array_map( array( $this, 'dependencies' ), array_intersect_key( $this->scripts, array_flip( $script['deps'] ) ) );
        }
    }

    private function _unset( $key, &$deps, &$out ) {
        $out[$key] = $this->scripts[$key];
        unset( $deps[$key] );
    }

    private function flattern( &$deps, &$out = array() ) {

        foreach( $deps as $key => $value ) {            
            empty($value) ? $this->_unset( $key, $deps, $out ) : $this->flattern( $deps[$key], $out );
        }
    }   

    function print_scripts() {

        if ( !$this->scripts ) return;

        $deps = array_map( array( $this, 'dependencies' ), $this->scripts );
        while ( $deps ) 
            $this->flattern( $deps, $js );

        foreach( $js as $key => $script ) {
            $script['data'] && printf( "<script> var %s = %s; </script>" . PHP_EOL, key( $script['data'] ), json_encode( current( $script['data'] ) ) );
            echo "<script id=\"$key-js\" src=\"$script[src]\" type=\"text/javascript\"></script>" . PHP_EOL;
        }
    }
}

Call to enqueue_script() function is for adding script, setting the souce and dependencies on other scripts, and additional data needed for the script.

$header = new mHeader();

$header->enqueue_script( 'jquery-ui', '//ajax.googleapis.com/ajax/libs/jqueryui/1.10.4/jquery-ui.min.js', array( 'jquery' ) );
$header->enqueue_script( 'jquery', '//ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js' );
$header->enqueue_script( 'custom-script', '//custom-script.min.js', array( 'jquery-ui' ), array( 'mydata' => array( 'value' => 20 ) ) );

$header->print_scripts();

And, print_scripts() method of the above example will send this output:

<script id="jquery-js" src="//ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js" type="text/javascript"></script>
<script id="jquery-ui-js" src="//ajax.googleapis.com/ajax/libs/jqueryui/1.10.4/jquery-ui.min.js" type="text/javascript"></script>
<script> var mydata = {"value":20}; </script>
<script id="custom-script-js" src="//custom-script.min.js" type="text/javascript"></script>

Regardless the fact that the script 'jquery' is enqueued after the 'jquery-ui', it is printed before because it is defined in 'jquery-ui' that it depend on 'jquery'. Additional data for the 'custom-script' are inside a new script block and are placed in front of it, it contains mydata object that holds additional data, now availible to 'custom-script'.

I have come out with an easy method to assign JavaScript variables using PHP.

It uses HTML5 data attributes to store PHP variables and then its assigned to JavaScript on page load.

Complete Tutorial can be found here

Example:

<?php

$variable_1 = "QNimate";
$variable_2 = "QScutter";

?>
    <span id="storage" data-variable-one="<?php echo $variable_1; ?>" data-variable-two="<?php echo $variable_2; ?>"></span>
<?php

Hers is the JS code

var variable_1 = undefined;
var variable_2 = undefined;

window.onload = function(){
    variable_1 = document.getElementById("storage").getAttribute("data-variable-one");
    variable_2 = document.getElementById("storage").getAttribute("data-variable-two");
}

Simply use one of the following methods.

<script type="text/javascript">
var js_variable  = '<?php echo $php_variable;?>';
<script>

OR

<script type="text/javascript">
    var js_variable = <?php echo json_encode($php_variable); ?>; 
</script>

As per your code

<$php
     $val = $myService->getValue(); // makes an api and db call
     echo '<span id="value">'.$val.'</span>';
$>

Now you can get value using DOM, use innerHTML of span id, in this case you dont need to do any call to server, or ajax or anyother thing.

Your page will print it using php, and you javascript will get value using DOM.

here is one i do not see posted as an option. it is similar to using ajax, but clearly different.

first, set a script's source directly to a PHP file.

<script type="text/javascript" src="url_to_your_php_file.php" /></script>

you could even pass a variable back to the PHP file such as this example:

<script type="text/javascript" src="url_to_your_php_file.php?var1=value1" /></script>

then in "your_php_file.php":

<?php
// THIS IS A SIMPLE EXAMPLE
// it demonstrates one method of using the src attribute to link
// to a PHP file which can generate javascripts dynamically
// and share data between PHP and javascript
// you may take this learning example and develop it further
// relying on your own coding skills for validating data
// and avoiding errors, of course
header( 'content-type: text/javascript' );

// if you pass a $_GET variable from the javascript
// you should add code to validate your $_GET variable(s)

// you can add code to query a database
// using $_GET['var1'] or some other criteria

// you can add simple variable assignments
$value = 'some value';

// for the OP's needs (assumes the class object has been defined)
$val = $myService->getValue();

?>
function name() {
    // pay attention because you need to use quotes properly
    // and account for possible quotes in the variable strings
    // to avoid both php and javascript errors
    // example assumes $val has been returned as a string
    // validate $val as needed using your method of choice
    var example1 = '<?php echo '"' . $val . '"'; ?>';
    var example2 = '<?php echo '"' . $value . '"'; ?>';
    var example3 = '<?php echo '"some other data"'; ?>';
    alert( example1 + ' / ' + example2 );
}
<?php
// you may even want to include additional files (.php or .js, etc)
@include 'local_path_to_some_other_js_file.js';
@include 'local_path_to_some_other_php_file.php';

exit;
?>

For those who have some problem using below code and it keep showing <?php echo $username?> or something like this.Go edit the httpd.conf in mime_module section by add this 'AddType application/x-httpd-php .html .htm' because it may disable by default.

   <?php
        $username = 1;
    ?>

    <script type="text/javascript">
        var myData = <?php echo $username ?>;
        console.log(myData);
        alert(myData);
    </script>

I'll assume that the data to transmit is a string.

As other commenters have stated, AJAX is one possible solution, but the cons outweigh the pros: it has a latency and it is harder to program (it needs the code to retrieve the value both server- and client-side), when a simpler escaping function should suffice.

So, we're back to escaping. json_encode($string) works if you encode the source string as UTF-8 first in case it is not already, because json_encode requires UTF-8 data. If the string is in ISO-8859-1 then you can simply use json_encode(utf8_encode($string)); otherwise you can always use iconv to do the conversion first.

But there's a big gotcha. If you're using it in events, you need to run htmlspecialchars() on the result in order to make it correct code. And then you haver to either be careful to use double quotes to enclose the event, or always add ENT_QUOTES to htmlspecialchars. For example:

<?php
    $myvar = "I'm in \"UTF-8\" encoding and I have <script>script tags</script> & ampersand!";
    // Fails:
    //echo '<body onload="alert(', json_encode($myvar), ');">';
    // Fails:
    //echo "<body onload='alert(", json_encode($myvar), ");'>";
    // Fails:
    //echo "<body onload='alert(", htmlspecialchars(json_encode($myvar)), ");'>";

    // Works:
    //echo "<body onload='alert(", htmlspecialchars(json_encode($myvar), ENT_QUOTES), ");'>";
    // Works:
    echo '<body onload="alert(', htmlspecialchars(json_encode($myvar)), ');">';

    echo "</body>";

However, you can't use htmlspecialchars on regular JS code (code enclosed in <script>...</script> tags). That makes use of this function prone to mistakes, by forgetting to htmlspecialchars the result when writing event code.

It's possible to write a function that does not have that problem, and can be used both in events and in regular JS code, as long as you enclose your events always in single quotes, or always in double quotes. Here is my proposal, requiring them to be in double quotes (which I prefer):

<?php

// Optionally pass the encoding of the source string, if not UTF-8
function escapeJSString($string, $encoding = 'UTF-8')
{
    if ($encoding != 'UTF-8')
        $string = iconv($encoding, 'UTF-8', $string);
    $flags = JSON_HEX_TAG|JSON_HEX_AMP|JSON_HEX_APOS|JSON_HEX_QUOT|JSON_UNESCAPED_SLASHES;
    $string = substr(json_encode($string, $flags), 1, -1);
    return "'$string'";
}

The function requires PHP 5.4+. Example usage:

<?php
    $myvar = "I'm in \"UTF-8\" encoding and I have <script>script tags</script> & ampersand!";
    // Note use of double quotes to enclose the event definition!
    echo '<body onload="alert(', escapeJSString($myvar), ');">';
    // Example with regular code:
    echo '<script>alert(', escapeJSString($myvar), ');</script>';
    echo '</body>';

Here is is the trick :

1. here is your 'PHP' to use that variable:

   <?php 
       $name = 'PHP variable';
       echo '<script>';
       echo 'var name = ' . json_encode($name) . ';';
       echo '</script>';
   ?>
   

2. Now you have a JS variable called 'name', and here is your 'Javascript' to use that variable :

   <script>
        console.log("i am everywhere " + name);
   </script>
   

    <?php 
    $your_php_variable= 22;
    echo "<script type='text/javascript'>var your_javascript_variable = $your_php_variable;</script>";  
    ?>

and that will work. It's just assigning a javascript variable and then passing the value of an existing php variable. Since php writes the javascript lines here, it has the value of of the php variable and can pass it directly.

let's say your variable is always integer, in that case this is easier

<?PHP
$number = 4;

echo '<script>';
echo 'var number = ' . $number . ';';
echo 'alert(number);';
echo '</script>';

?>

output:

<script>var number = 4;alert(number);</script>

let's say your variable is not an integer, but if you try above method you will get something like this:

<script>var number = abcd;alert(number);</script>

but in javascript this is a syntax error.

so in php we have a function call json_encode that encode string to a json object.

<?PHP
$number = 'abcd';

echo '<script>';
echo 'var number = ' . json_encode($number) . ';';
echo 'alert(number);';
echo '</script>';

?>

since abcd in json it's "abcd",so it looks like this:

<script>var number = "abcd";alert(number);</script>

You can use same method for arrays:

<?PHP
$details = [
'name' => 'supun',
'age' => 456,
'weight' => '55'
];

echo '<script>';
echo 'var details = ' . json_encode($details) . ';';
echo 'alert(details);';
echo 'console.log(details);';
echo '</script>';

?>

and your javascript looks like this:

<script>var details = {"name":"supun","age":456,"weight":"55"};alert(details);console.log(details);</script>

console output

This is how I do it (there are many ways):

1. Convert the data into JSON
2. Call AJAX to recieve JSON file
3. Convert JSON into Javascript object

Example:

STEP 1

<?php

   $servername = "localhost";
   $username = "";
   $password = "";
   $dbname="";
   $conn = new mysqli($servername, $username, $password,$dbname);

   if ($conn->connect_error) {
      die("Connection failed: " . $conn->connect_error);
   } 

   $sql = "SELECT id, name ,image FROM phone";
   $result = $conn->query($sql);

   while($row =$result->fetch_assoc()){ 
      $v[]=$row;    
   }

  echo json_encode($v);

  $conn->close();
?>

STEP 2

function showUser(fnc) {
   var xhttp = new XMLHttpRequest();

   xhttp.onreadystatechange = function() {
      if (this.readyState == 4 && this.status == 200) {
         // STEP 3    
         var p=JSON.parse(this.responseText);}
       }
    }

After many research, I found the easiest method to pass all kind of variables easily.

In your php script, you have two variable, and you are trying to send them to your js scripts :

$php_var1 ="Hello world;
$php_var2 ="Helloow";
echo '<script>';
echo 'var js_variable1= ' . json_encode($php_var1) . ';';
echo 'var js_variable2= ' . json_encode($php_var2) . ';';
echo '</script>';

In any of your javascript called on your page, simply call the vars.

Hope I helped anyone !