This is fairly well-trod territory:

http://www.google.com/search?q=why+php+sucks

A good resource for comparing languages is the shootout:

http://shootout.alioth.debian.org/gp4sandbox/benchmark.php?test=all&lang=all

Most people just refer to the overall scores, but what's more interesting is seeing the same program done in different languages.

This one is pretty straightforward:

http://shootout.alioth.debian.org/gp4sandbox/benchmark.php?test=nbody&lang=php&id=0

Compare to the same thing in Python, Ruby, Scheme, Common Lisp, and Haskell:

http://shootout.alioth.debian.org/gp4sandbox/benchmark.php?test=nbody&lang=python&id=0

http://shootout.alioth.debian.org/gp4sandbox/benchmark.php?test=nbody&lang=ruby

http://shootout.alioth.debian.org/gp4sandbox/benchmark.php?test=nbody&lang=chicken&id=3

http://shootout.alioth.debian.org/gp4sandbox/benchmark.php?test=nbody&lang=sbcl

http://shootout.alioth.debian.org/gp4sandbox/benchmark.php?test=nbody&lang=ghc