With respect to web site programming, I can't think of anything off-hand I'd want to do with one that I couldn't do with the other. I personally find it easier and more convenient to use PHP for web site programming. On the other hand, as Perl was designed as a more general purpose language while PHP was designed from the ground up with web programming in mind, I usually find it easier to use Perl for non-web-related programming.
If you google on something like "comparison of PHP and Perl", you'll find lots of pages. If you go to a page on a site that has the word "perl" in the domain name, it will probably tell you why Perl is much better; and if the domain name has "php" in it, guess what: it will probably tell you PHP is better. 🙂 Try reading some of each, and then form your own opinion based on what it is you want out of a programming language along with your personal tastes in programming style and technique.