How to call external javascript function inside a php tag?

How to call external javascript function inside a php tag?

I have a php tag and inside that i have a script tag like this ..
<?php
echo' <script language= javascript type= text/javascript>
//HERE
</script>';
?>
I need to call external javascript function(External means external .js
file) and this is a .php page and have not I know this is a stupid
question but plz give me a solution??