Checks the plugins directory and retrieve all plugin files with plugin data.

WordPress only supports plugin files in the base plugins directory
(wp-content/plugins) and in one directory above the plugins directory
(wp-content/plugins/my-plugin). The file it looks for has the plugin data
and must be found in those two locations. It is recommended to keep your
plugin files in their own directories.

The file with the plugin data is the file that will be included and therefore
needs to have the main execution for the plugin. This does not mean
everything must be contained in the file and it is recommended that the file
be split for maintainability. Keep everything in one file for extreme
optimization purposes.

function get_plugins( $plugin_folder = '' ) {

	$cache_plugins = wp_cache_get( 'plugins', 'plugins' );
	if ( ! $cache_plugins ) {
		$cache_plugins = array();
	}

	if ( isset( $cache_plugins[ $plugin_folder ] ) ) {
		return $cache_plugins[ $plugin_folder ];
	}

	$wp_plugins  = array();
	$plugin_root = WP_PLUGIN_DIR;
	if ( ! empty( $plugin_folder ) ) {
		$plugin_root .= $plugin_folder;
	}

	// Files in wp-content/plugins directory.
	$plugins_dir  = @opendir( $plugin_root );
	$plugin_files = array();

	if ( $plugins_dir ) {
		while ( ( $file = readdir( $plugins_dir ) ) !== false ) {
			if ( '.' === substr( $file, 0, 1 ) ) {
				continue;
			}

			if ( is_dir( $plugin_root . '/' . $file ) ) {
				$plugins_subdir = @opendir( $plugin_root . '/' . $file );

				if ( $plugins_subdir ) {
					while ( ( $subfile = readdir( $plugins_subdir ) ) !== false ) {
						if ( '.' === substr( $subfile, 0, 1 ) ) {
							continue;
						}

						if ( '.php' === substr( $subfile, -4 ) ) {
							$plugin_files[] = "$file/$subfile";
						}
					}

					closedir( $plugins_subdir );
				}
			} else {
				if ( '.php' === substr( $file, -4 ) ) {
					$plugin_files[] = $file;
				}
			}
		}

		closedir( $plugins_dir );
	}

	if ( empty( $plugin_files ) ) {
		return $wp_plugins;
	}

	foreach ( $plugin_files as $plugin_file ) {
		if ( ! is_readable( "$plugin_root/$plugin_file" ) ) {
			continue;
		}

		// Do not apply markup/translate as it will be cached.
		$plugin_data = get_plugin_data( "$plugin_root/$plugin_file", false, false );

		if ( empty( $plugin_data['Name'] ) ) {
			continue;
		}

		$wp_plugins[ plugin_basename( $plugin_file ) ] = $plugin_data;
	}

	uasort( $wp_plugins, '_sort_uname_callback' );

	$cache_plugins[ $plugin_folder ] = $wp_plugins;
	wp_cache_set( 'plugins', $cache_plugins, 'plugins' );

	return $wp_plugins;
}