IntelliJ IDEA has the functionality of all of their other products except appCode available as plugins, but the plugins aren't necessarily as up-to-date as the standalone products. The plugin version usually gets updated a couple months after a major version of the standalone product is released, in my experience.
For example, the plugin list for my copy of IntelliJ Ultimate 11 (cheap upgrade to 12 incoming, woo) shows a PHP plugin whose description says "PhpStorm/WebStorm 4.0.3 version". PhpStorm 5 was released in September, but a lot of the IntelliJ plugins for its features haven't seen an update since then.
Edit: Upgraded to IDEA 12, and it looks like the bundled plugins contain full WebStorm 5 functionality.
For example, the plugin list for my copy of IntelliJ Ultimate 11 (cheap upgrade to 12 incoming, woo) shows a PHP plugin whose description says "PhpStorm/WebStorm 4.0.3 version". PhpStorm 5 was released in September, but a lot of the IntelliJ plugins for its features haven't seen an update since then.
Edit: Upgraded to IDEA 12, and it looks like the bundled plugins contain full WebStorm 5 functionality.