PhpStorm is an IDE (integrated development environment) specifically built for PHP developers. IDEs make programming much easier, by providing handy features such as autocomplete, code hinting, boilerplate code and more, all aimed at increasing programmer productivity. PhpStorm takes away as much of the necessary mundane programming tasks as possible, so that programmers can concentrate on their application.
How is PhpStorm useful?
PhpStorm is not free and can actually be quite costly when there are less fully featured, but free alternatives. However, for serious PHP developers and organisations the features on offer will probably justify the cost. Less time spent wrestling with unwieldy code bases is more time writing code that matters.