I created a PHP file containing various syntax and logic errors and tested it in several editors. PhpStorm flagged 23 errors, most of which were not caught by the other editors. It will show undeclared variables, variables that are set but never used, function argument count mismatches, duplicate declarations of classes (even if they're in different files), variables that are set but not used before their value is changed, things like using a function call inside empty(), and a ton of other subtle errors that would be very difficult to find. Every file with a serious PHP error shows up with a red underline in the file tree. The error checking is tuned to your particular version of PHP.
Not to mention being able to instantly jump to (and return from) a method call to the code of the method --- no matter where it is in your project.
I wish I could get a kickback for recommending it.