Fix issue introduced previously whereby php_Self is now used unchecked.

introduced previously by john attempting to fix symlinks. Since we now use php 5.2, we can make use of filter_var.

This is a simpler version of what we were trying to do previously aka http://git.mantisforge.org/w/mantisbt.git?a=commitdiff;h=5ac1fdf32717d0c82cca7e7660dd4fd316a6a1b8

Depending on server/mantis config this can lead to XSS issues