Provide plugin and theme directories as built-in variables

features/behat-steps.feature

@@ -468,6 +468,45 @@ Feature: Test that WP-CLI Behat steps work as expected
     When I launch in the background `wp eval 'sleep(2); echo "done";'`
     # Background process should not block, continuing immediately
 
+  @require-wp
+  Scenario: Use the plugin and theme directories of a WP installation
+    Given a WP installation
+    Then the {PLUGIN_DIR}/akismet directory should exist
+    And the {THEME_DIR} directory should exist
+
+  # Skipped on Windows because WordPress mixes path separators in these paths there.
+  @require-wp @skip-windows
+  Scenario: Plugin and theme directory variables match the WordPress constants
+    Given a WP installation
+    When I run `wp eval 'echo WP_PLUGIN_DIR;'`
+    Then STDOUT should be:
+      """
+      {PLUGIN_DIR}
+      """
+
+    When I run `wp eval 'echo get_theme_root();'`
+    Then STDOUT should be:
+      """
+      {THEME_DIR}
+      """
+
+  # Skipped on Windows because WordPress mixes path separators in these paths there.
+  @require-wp @skip-windows
+  Scenario: Plugin and theme directory variables follow a custom wp-content directory
+    Given a WP installation
+    And a custom wp-content directory
+    When I run `wp eval 'echo WP_PLUGIN_DIR;'`
+    Then STDOUT should be:
+      """
+      {PLUGIN_DIR}
+      """
+
+    When I run `wp eval 'echo get_theme_root();'`
+    Then STDOUT should be:
+      """
+      {THEME_DIR}
+      """
+
   @require-wp
   Scenario: Test custom wp-content directory
     Given a WP installation
@@ -479,6 +518,8 @@ Feature: Test that WP-CLI Behat steps work as expected
       """
       WP_CONTENT_DIR
       """
+    And the {PLUGIN_DIR} directory should exist
+    And the {THEME_DIR} directory should exist
 
   @require-wp
   Scenario: Test WP multisite installation
@@ -525,6 +566,18 @@ Feature: Test that WP-CLI Behat steps work as expected
     When I run `wp core version`
     Then STDOUT should not be empty
 
+    When I run `wp eval 'echo WP_PLUGIN_DIR;'`
+    Then STDOUT should be:
+      """
+      {PLUGIN_DIR}
+      """
+
+    When I run `wp eval 'echo get_theme_root();'`
+    Then STDOUT should be:
+      """
+      {THEME_DIR}
+      """
+
   # Skipped on Windows because of curl getaddrinfo() errors.
   @require-wp @require-composer @skip-windows
   Scenario: Test WP installation with Composer and custom vendor directory

src/Context/FeatureContext.php

@@ -165,7 +165,7 @@ class FeatureContext implements Context {
 
 	/**
 	 * Array of variables available as {VARIABLE_NAME}. Some are always set: CORE_CONFIG_SETTINGS, DB_USER, DB_PASSWORD, DB_HOST, SRC_DIR, CACHE_DIR, WP_VERSION-version-latest.
-	 * Some are step-dependent: RUN_DIR, SUITE_CACHE_DIR, COMPOSER_LOCAL_REPOSITORY, PHAR_PATH. One is set on use: INVOKE_WP_CLI_WITH_PHP_ARGS-args.
+	 * Some are step-dependent: RUN_DIR, SUITE_CACHE_DIR, COMPOSER_LOCAL_REPOSITORY, PHAR_PATH, and PLUGIN_DIR and THEME_DIR for the WordPress files in RUN_DIR. One is set on use: INVOKE_WP_CLI_WITH_PHP_ARGS-args.
 	 * Scenarios can define their own variables using "Given save" steps. Variables are reset for each scenario.
 	 *
 	 * @var array<string, string>
@@ -1849,6 +1849,10 @@ public function download_wp( $subdir = '', $version = '' ): void {
 
 		self::copy_dir( self::$cache_dir, $dest_dir );
 
+		if ( ! $subdir ) {
+			$this->set_content_dir_variables( $dest_dir . '/wp-content' );
+		}
+
 		if ( ! is_dir( $dest_dir . '/wp-content/mu-plugins' ) ) {
 			mkdir( $dest_dir . '/wp-content/mu-plugins', 0777, true /*recursive*/ );
 		}
@@ -2048,6 +2052,21 @@ public function install_wp_with_composer( $vendor_directory = 'vendor' ): void {
 		}
 
 		$this->proc( 'wp core install', $install_args )->run_check();
+
+		$this->set_content_dir_variables( $this->variables['RUN_DIR'] . '/WordPress/wp-content' );
+	}
+
+	/**
+	 * Expose the plugins and themes directories as {PLUGIN_DIR} and {THEME_DIR}.
+	 *
+	 * Matches the output of `wp plugin path` and `wp theme path`.
+	 *
+	 * @param string $content_dir
+	 * @param string $plugin_dir Defaults to the plugins directory inside $content_dir.
+	 */
+	public function set_content_dir_variables( $content_dir, $plugin_dir = '' ): void {
+		$this->variables['PLUGIN_DIR'] = $plugin_dir ?: $content_dir . '/plugins';
+		$this->variables['THEME_DIR']  = $content_dir . '/themes';
 	}
 
 	public function composer_add_wp_cli_local_repository(): void {

src/Context/GivenStepDefinitions.php

@@ -470,6 +470,8 @@ public function given_a_custom_wp_directory(): void {
 
 		file_put_contents( $wp_config_path, $wp_config_code );
 
+		$this->set_content_dir_variables( $this->variables['RUN_DIR'] . '/my-content', $this->variables['RUN_DIR'] . '/my-plugins' );
+
 		if ( 'sqlite' === self::$db_type ) {
 			$db_dropin = $this->variables['RUN_DIR'] . '/my-content/db.php';