[FX.php List] Validating a URL

[Edward L. Ford]

Hi,
I have a PHP submission form to FileMaker that does some pretty  
rigorous data validation to ensure good data.  I'd like to extend  
this a little further to validating that a URL actually exists.

Essentially, what I want to disallow is a URL that leads to a 404  
error page, or if it returns 200 (OK).  How might I validate this in  
PHP?  All suggestions are welcomed.

--Ed