I run apache in a chrooted environement already so this may not apply to everyone since I will not be discussing how to chroot apache, just php.
Install PHP5 as normal on you system and then carry out the following.
cp /usr/lib/apache/1.3/libphp5.so [chroot directory]/usr/lib/apache/1.3/libphp5.so
cp /usr/lib/libxml2.so.2 [chroot directory]/usr/lib/libxml2.so.2
Its as simple as that.