Include PHP inside JavaScript (.js) files

7 years later update: This is terrible advice. Please don't do this.

If you just need to pass variables from PHP to the javascript, you can have a tag in the php/html file using the javascript to begin with.

<script type="text/javascript">
    var phpVars = <?php echo json_encode($vars) ?>;
</script>
<script type="text/javascript" src="yourScriptThatUsesPHPVars.js"></script>

If you're trying to call functions, then you can do this like this

<script type="text/javascript" src="YourFunctions.js"></script>
<script type="text/javascript">
    // assume each element of $arrayWithVars has already been json_encoded
    functionOne(<?php echo implode(', ', $arrayWithVars); ?>);

    functionTwo(<?php echo json_encode($moreVars) ?>, <?php echo json_encode($evenMoreVars) ?>);
</script>

AddType application/x-httpd-php .js

AddHandler x-httpd-php5 .js

<FilesMatch "\.(js|php)$">
SetHandler application/x-httpd-php
</FilesMatch>

Add the above code in .htaccess file and run php inside js files

DANGER: This will allow the client to potentially see the contents of your PHP files. Do not use this approach if your PHP contains any sensitive information (which it typically does).

If you MUST use PHP to generate your JavaScript files, then please use pure PHP to generate the entire JS file. You can do this by using a normal .PHP file in exactly the same way you would normally output html, the difference is setting the correct header using PHP's header function, so that the correct mime type is returned to the browser. The mime type for JS is typically "application/javascript"