When you're including <script src="...."> files you are actually including client-side scripts.
In the remotely hosted client-side script, you can have PHP within that, however you will need to output a client-side script!
Here's a quick example;
Remotely hosted script: (dyn-jscript.php)
<?php
if($_GET['page'] =='news') {
echo "document.write(\"This is the news page...\")";
}
else {
echo "document.write(\"This is another page...\")";
}
?>
Static html page: (dyn.html)
<html>
...
<script language="JavaScript" type="text/javascript" src="dyn-jscript.php?page=news"></script>
...
</html>
This will output "This is the news page..."
Good luck.