All Hail Valgrind!

Valgrind Tools

Also make sure you use an atomic file deployment mechanism like rsync or mv. Never tar or cp files to their final destinations on a running production server.

Beware of cache slams

When a file is modified on a busy server there is a race to compile and cache the new file. This can put a drain on your server and its resources. Because APC uses the device+inode as the hash, you can make use of that in your file update strategy. Write new files to a temporary filename or perhaps a temporary directory, hit it once, then rename it to its publically visible location. mtime does not change on a rename as long as you are not moving it from one fs to another.