Hello:
First, PSA uses the PHP rpm's that are distributed with the OS. You can, however, make use of whatever RPM's you'd like. I should note, that certain things like webmail may care about PHP, so perform these upgrades at your own risk.
Next, you should bookmark
www.atomicrocketturtle.com (aka. ART). Very nice resource for PSA tweaks and RPM's that you can use to customize your PSA installation.
As for PHP, ART has RPM's for php 4.3.11 on RH 9. They are located here:
http://www.atomicrocketturtle.com/atomic/art/9/RPMS/.
Download php-4-3-11-8.rh90.art.i386.rpm and whatever php extensions you need. If you do an rpm -qa |grep php on your server, you can get a list of what is currently installed - you will at least need the newer versions of these RPM's in order to upgrade.
Once these RPM's are on your server, you can perform an rpm -Uvh php*.rpm in the directory where you placed these php RPM's. You will need to restart Apache (service httpd restart or /etc/init.d/httpd restart or /etc/rc.d/init.d/httpd restart - whatever floats your boat) in order for the new version to become active. You should test after you perform the upgrade. It would also be a good idea to have the original RPM's around in case you need to revert for any reason. It would also be a good idea to make backups of /etc/php.ini and /etc/php.d/.
I would recommend checking out ART as he has other mechanisms you can use, namely using yum to do these type of upgrades. Regardless, this should get you started.
Good luck.
Steve