Ok, etch is oldstable. I would wager that php5.3 in experimental isn't meant to be able to run on Etch. Perhaps not even Lenny (stable).
Picking one package
.. I'm mostly right. libxml2 (>= 2.6.28) is not available in Etch but is in Lenny.
You can either upgrade your system to Lenny, or attempt to build php5 from experimental on your Etch system. With all the dependencies it probably isn't worth it, but you can try something like:
apt-get install build-essential
apt-get build-dep php5
apt-get source -b -t experimental php5
And see if it will build. If it does,
dpkg -i php5-somethingsomething.deb