I've asked the same question before, and the answers I've received are basically:
A) People like to compare frameworks (e.g. Rails) to the PHP programming language, rather than an apples-to-apples comparison, and therefore PHP always falls short in their eyes.
B) The abundance of poorly written, insecure legacy code has besmirched PHP's reputation beyond repair, even if its actual security isn't bad.
C) The fact that n00bs can pick it up quickly and get a dynamic website out of PHP with minimal effort leads elitists to look down on it from their glass enclaves.
D) Academic reasons that no one outside academia cares about.
Full disclosure: I read and write PHP every day regardless of what people say about it. :D
A) People like to compare frameworks (e.g. Rails) to the PHP programming language, rather than an apples-to-apples comparison, and therefore PHP always falls short in their eyes.
B) The abundance of poorly written, insecure legacy code has besmirched PHP's reputation beyond repair, even if its actual security isn't bad.
C) The fact that n00bs can pick it up quickly and get a dynamic website out of PHP with minimal effort leads elitists to look down on it from their glass enclaves.
D) Academic reasons that no one outside academia cares about.
Full disclosure: I read and write PHP every day regardless of what people say about it. :D