500 Internal Server Error

Hello All, What is the situation when we get internal server error 500 on PHP pages? From Internet I got some info like, you get it when : 1) If friendly urls are not supported by apache.(mod_rewrite) 2) If max_execution_time max's out. and some more related to apache. I got a weird situation where I do PHP eval() and if the parsed string is wrong, I get internal server error 500. So question is I was under the impression that PHP code errors will never result in a http response errors. Am I completely wrong here? Can anyone tell me from their experience what are the specific scenarios we get this error? Thanks, V