Red Hats website lists It lists it as a package.. php-devel-4.2.2-17.i386.rpm however I can not find it using up2date or cPanels software installer, may be because of the channel it's in. But red hat has a link to download it. to install it, download it to your server, then do:
rpm -i php-devel-4.2.2-17.i386.rpm
And that should install it for you. Note that up2date does not know about this RPM and can therefor not update it later on.
Sorry for not posting the link, but the board won't let me do that yet!