It's useless to use the latest greatest feature set in PHP or MySQL, if the organizations you're targeting are all running RHEL 4u8, 5u2 or even RHEL3! And many do. The Enterprise Distributions for Linux lag behind public version for up to 18 months! And they won't easily upgrade just because you ask or because your application was built using the most recent version of an open source package. The company may be able to give you a special server in some DMZ but you may lack access to certain key servers, because the security status of your app and server have not (yet) been approved.
Instead, focus writing apps using a stable, common, proven API. Don't use a recent function just because it's there, but only if you need it or because it is simply very much more efficient in usage than using an older call, function or feature.
That being said, I have always limited our customers to whatever PHP or MySQL version was bundled into our releases. As an exception they could use Oracle's patches from a more recent but supported channel. But never a fresh version built from source off of the Internet. So I was pleasantly surprised to see that Oracle offers some pre-built - unsupported - RPMs for OEL4 and OEL5 in case you need a more recent version than those in the ULN channels: Oracle: PHP RPMs.
It may also be good to know that Oracle now also bundles Zend inside their ULN channels. Check that if you write heavy-use web application using a LAMP stack on Oracle Linux. In addition, Oracle has a PHP extension "PHP OCI8" that is ready to go for using an Oracle database instead of a MySQL database, and a howto on installing a LAOP stack (Linux, Apache, oracle db, PHP).