When installing a source RPM using "rpm -i" then the sources are copied to the relevant directories (/usr/src/redhat/SOURCES) and a package specification file is put in /usr/src/redhat/SPECS. This may be a bit different on Mandrake.
Nothing else happens. You have to re-build the source RPM package before it can be installed in a useful state. For that, you go to the directory containing the "php.spec" file and run:
rpm -bb php.spec
See the RPM manual and this URL for more information:
http://rpmdp.org/rpmbook/