I notice that I can substitue PHP into JavaScript that's place inline in the HTML, as below:
<script>
jQuery('select#age_group').val(<?php echo $age_group?>);
</script>
But the PHP substitution doesn't work inside a .js file that the PHP file loads. Why is that and how can I do the substituion in the loaded .js file?
Here are two ways you can do this without changing the file extensions that the PHP engine can process.
First, you can set a global variable inline and then access that variable in the JS you include.
<script>global_age_group = <?php echo $age_group?>;</script>
<script src="js_that_accesses_age_group.js" /></script>
(standard warnings about global variables apply)
Second, you can give your JS file a .php extension, so...
<script src="js_that_accesses_age_group.php" type ='application/javascript'/></script>
And then use
<?php echo $age_group?>
in that file as you do in your inline example.
Try this instead... Let me know if it works
</form>
</div>
<?php echo"<script>
jQuery('select#age_group').val($age_group);
</script>";
?>
</body>
</html>
you pass the value on a element attribute or on js var as you did
<script>
var age = 20;
</script>
<script src="main.js"></script>
and your can use the age inside main.js
By default, PHP will only process files ending in certain extensions (most commonly .php). If you want to use PHP in a JavaScript file then the easiest way to do it is to give it a .php extension.
Most browsers will also refuse to process your JavaScript unless it has the right content type, so at the top of your file add
<?php
header("Content-Type: application/javascript");
The Simple answer that you can't !
You can't write a PHP script any where except .php files, That you can't write php scripts in .html, .css or .js files because the compiler will not pass .js file or .html file to read it!
.jsto the file extensions PHP processes. Presumably this is part of yourhttpd.conf