Brew doctor: dyld: Library not loaded & Error: No available formula for zlib

When I brew doctor I get the following errors:

dyld: Library not loaded: /usr/lib/libltdl.7.dylib
  Referenced from: /usr/local/bin/php
  Reason: image not found
Error: No available formula for zlib

The file libltdl.7.dylib is not in my /usr/lib directory, but there are several other .dylib files.

I'm running 10.8.1 with the latest version of Xcode (4.4.1) and its Command Line Tools installed.

Any idea how to solve these errors?