How do I embed PHP code in JavaScript?

If you put your JavaScript code in the PHP file, you can, but not otherwise. For example:

page.php (this will work)

function jst()
{
    var i = 0;
    i = <?php echo 35; ?>;
    alert(i);
}

page.js (this won't work)

function jst()
{
    var i = 0;
    i = <?php echo 35; ?>
    alert(i);
}

If your whole JavaScript code gets processed by PHP, then you can do it just like that.

If you have individual .js files, and you don't want PHP to process them (for example, for caching reasons), then you can just pass variables around in JavaScript.

For example, in your index.php (or wherever you specify your layout), you'd do something like this:

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

You could then use my_var in your JavaScript files.

This method also lets you pass other than just simple integer values, as json_encode() also deals with arrays, strings, etc. correctly, serialising them into a format that JavaScript can use.

PHP has to be parsed on the server. JavaScript is working in the client's browser.

Having PHP code in a .js file will not work, except you can tell the server to parse the file you want to have as .js before it sends it to the client. And telling the server is the easiest thing in the world: just add .php at the end of the filename.

So, you could name it javascript.php. Or, so you know what this file is PRIMARILY, you could name it javascript.js.php - the server will recognize it as .php and parse it.

This is the bit of code you need at the top of your JavaScript file:

<?php
    header('Content-Type: text/javascript; charset=UTF-8');
?>

(function() {
    alert("hello world");
}) ();