Inconsistency, very prone to security mistakes, poor type system, and some people just find its syntax "ugly." For example lots of folks hate $strings and $_WTF['bbq'] type syntax. Perl is another language whose syntax draws a lot of haters.

I'd say the second problem is the most objectively bad. It is very easy to write PHP code that exposes you to all kinds of known exploit classes like SQL injection, command injection, cross-site scripting, etc. The language seems to encourage poor practices like building query strings directly from remote input without the use of a true query builder.

PHP also has a problem that it shares with JavaScript and to a lesser extent Java -- the quality of the user base and user-contributed code is often very poor. If you look for PHP code on StackOverflow or Reddit or similar sites, you are likely to get something that is horribly wrong, comically inefficient, or whose use will immediately introduce a dangerous remote security vulnerability. This comes from the simple fact that it's often a beginner language, so you have a lot of inexperienced people participating in forums and contributing sample code.