Basically: PhpStorm = WebStorm + PHP + Database support
At the same time: plugins that require PHP support would not be able to install in WebStorm (for obvious reasons).
P.S. Since WebStorm has different release cycle than PhpStorm, it can have new JS/CSS/HTML oriented features faster than PhpStorm (it’s all about platform builds used).
For example: latest stable PhpStorm is v7.1.4 while WebStorm is already on v8.x. But, PhpStorm v8 will be released in approximately 1 month (accordingly to their road map), which means that stable version of PhpStorm will include some of the features that will only be available in WebStorm v9 (quite few months from now, lets say 2-3-5) — if using/comparing stable versions ONLY.
UPDATE (2016-12-13): Since 2016.1 version PhpStorm and WebStorm use the same version/build numbers .. so there is no longer difference between the same versions: functionality present in WebStorm 2016.3 is the same as in PhpStorm 2016.3 (if the same plugins are installed, of course).
Everything that I know atm. is that PHPStorm doesn’t support JS part like Webstorm
UPDATE (2016-12-13): Here is the list of plugins that are bundled with WebStorm 2016.3 but require manual installation in PhpStorm 2016.3 (if you need them, of course):
- Java Server Pages (JSP) Integration
- PhoneGap/Cordova Plugin
- Polymer & Web Components
- Pug (ex-Jade)
- Stylus support