It used to have some really strangely behaving functions and syntax. Stuff like inconsistency between throwing and returning error codes, weirdly named standard functions, adding strings with numbers, really really strange but very well known bugs you had to constantly work around and a lots of ambiguous syntax causing strange bugs rather than just crashing to help you out. It also had this horrible feature where if you write $$variable where the $variable contains a string, it'd access the contents of the variable with that string as name, which allows for huge security risks but also lots of "quirky" coding.

It was also one of the most supported languages for web back-ends, so lots of people had the displeasure of working with it.

But AFAIK the newer versions have made it somewhat of a normal language. r/lolphp used to document a lot of those oddities, so you could sort it by top all time if you want to see some weird shit.