PHP is deliberately specialized for web pages. This theoretically makes it easier to develop web interfaces quickly using PHP. It's not meant for general pattern processing.
The PHP interpreter is written directly in C. Perl runs (after compilation) as a bytecode interpreter. I think that this is the other main advantage of PHP: response time should be improved by using one layer of interpretation instead of two. I'm not familar with the internal data representation of PHP, so I can't immediately compare the two languages that way.
Unfortunately, PHP's strength (interpreter written in C) is also its critical downfall, from a network security perspective. I have advised two clients not to use PHP for publicly exposed scripts because of repeated "arbitrary code execution" buffer overflow bug reports in PHP's POST parameter parsing code...I think I've seen at least three distinct reports of this in 2002. So far, both clients have taken my advice.