Some later versions of PHP require a newer version of libxml2 than comes on the earlier versions of RedHat. On the other hand, earlier versions seem to be OK without it. Furthermore, you never know when the PHP winkies will fix this problem so you might just want to try it without, first.
If you do decide that you need libxml2, download the tar file from http://xmlsoft.org/downloads.html and untar it in the top level source directory (e.g. /rpm/Apache):
tar -xvzf libxml2-a.b.yy.tar.gz
It will create a new directory for that version of libxml2. Switch to that directory and build libxml2. Note that the Python portion of libxml2 is likely to cause the build to crap out so, unless you really need Python support, you should disable it:
cd libxml2-a.b.yy
./configure --prefix=/usr/local --without-python
make
The fact that Python is broken probably precludes overwriting the old libxml2 in /usr/lib, hence the reason for installing it in /usr/local/lib. This will allow libxml2 to be built but won't wipe out other things that use Python (like GNOME).
Switch to super-duper user and install libxml2:
su
make install