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PhpStorm comes with a large number of plugins bundled. This means that they are installed and activated after installing PhpStorm. We can see these plugins from the IDE Settings | Plugins (press Ctrl+Alt+S or CMD+, on Mac OS X). For example, the Database and SQL Editor functionality in PhpStorm is built as a plugin which comes bundled with PhpStorm.
We can add functionality to our IDE by installing plugins. These plugins can be JetBrains plugins, developed by JetBrains, or third-party plugins.
Install JetBrains plugin
From the IDE Settings | Plugins, we can click Install JetBrains plugin... to browse all plugins that are developed by JetBrains. This plugin repository contains various plugins that enrich PhpStorm. Doing AngularJS development? Why not install the plugin for that?
Many additional features can be enabled from here. NodeJS support, LiveEdit, Spy-JS, Meteor, VIM-style editing with IdeaVim, ... Once we found the plugin we want to add, clicking Install plugin will download and install the plugin. Note that after plugin installation, the IDE has to be restarted to be able to use it.
Using Browse Repositories... (under IDE Settings | Plugins), we can browse third-party plugins for PhpStorm. A web-based gallery is available as well.
Some plugins are small additions, like Copy on Steroids which lets us copy code as rich-text or FavoriteFolders which lets us pin folders we often use. Other plugins are very extensive and add a lot of functionality, like the Symfony2 plugin.
Install plugin from disk
Plugins can be installed from disk using the Install plugin from disk... button. We can pick the path to the plugin, shipped as a ZIP or JAR file, and then load it into our IDE.
To uninstall a plugin, we can open the IDE Settings | Plugins and click the Uninstall plugin button. Note that not all plugins can be uninstalled: some are required for PhpStorm to function properly. Also some plugins may depend on other plugins.