Common problem. Try:
locate libmysqlclient.so
and see where that file is. If nothing comes up, su - to root and run /etc/cron.daily/slocate to force the locate db to update.
Whereever that file is, you need to edit /etc/ld.so.conf and add a line that describes the path to libmysqlclient.so. Let's say the result of the locate command looked like this:
/usr/local/lib/mysql/libmysqlclient.so
Then you would enter a new line in /etc/ld.so.conf that was
/usr/local/lib/mysql
Then save the file and run ldconfig
Now apache should load with php et. al.