JS Javascript into a single node / page
Sounds to me like you're trying to call the drupal_add_js() function from within a node edit form (or maybe the body of a form).  Instead of that, just change the body's input filter to HTML (or whatever filter you have that allows &lt;script&gt; tags) and call the external JavaScript there as you would in any HTML page.  You don't need to use PHP for that. 

I would add that that probably isn't best practice.  Using drupal_add_js() is, but you should use drupal_add_js() in page templates (e.g., node.tpl.php) or in preprocessor functions (e.g., hook_preprocess_node()).

As an aside: be very careful of the PHP input filter, it has major security implications.