Capturing The Output Of A Script Src Tag

I'm sure you all have seen how using the src argument in a script tag
is essentially the same as including the body of the script within
your page. For example:

<script type="text/javascript"
src="http://somepage.com/somead/show_ads.js">
</script>

Is there a way to capture the html output of that script and save it
to a variable or text file so that I can use a language like php to
parse it and look for certain words or phrases?

For instance, this particular url executes some javascript code which
produces an iframe on my page, but view source only displays the
script tag. Using innerHTML I can get the iframe code to print in
another browser window, but I would prefer the iframe html be save to
a variable or text file so that I can write a script to parse it on
the fly.