Compile PHP5 yourself... try using this string (assuming you have gd, libjpeg/png, zlip and freetype already installed).
./configure --prefix=/Library/php5 \
--with-zlib \
--with-xml \
--with-gd \
--with-jpeg-dir=/usr/local \
--with-png-dir=/usr/local \
--with-mysql=/usr/local/mysql \
--with-apxs2=/Library/Apache2/bin/apxs \
--with-libxml-dir=/usr/local
The most important for you is that php5 is installed in it's own directory (I chose /Library/php5): --prefix=/Library/php5
So the correct procedure is this:
(run the above ./configure script with any modifications you might need - may need to sudo it)
sudo make
sudo make install
I'm also running tiger and got this up in a matter of minutes.