Check which php functions will work in a specific php version
<p>At least this shows I'm not afraid to look stupid.  Here it goes:
At some point while writing my php project, I determined that it would need php verion 5.2 to work properly.  I didn't document why.  Now, a user is asking me what will break if they run my app on 5.1 and I simply can't remember.</p>

<p>Is there any automated way to check which functions will fail using 5.1.  I looked at PHP_CodeSniffer, but I'm not sure that will do what I want.  Any thoughts?</p>
