Reinstall/repair Apache Installation from base system

Hi,

I use Snow Leopard and I was not aware of the fact that php and apache2 comes with the base installation. I delete now some .so files like /usr/libexec/apache/mod authnfile.so, so I'm not able to start apache anymore.

How can I reinstall apache so that the deleted files will be restored?

Thanks for help,
Matthias