d_settings_error' ) ) {
					add_settings_error(
						// Slug title of the setting.
						$this->group_name,
						// Suffix-ID for the error message box. WordPress prepends `setting-error-`.
						$key,
						sprintf(
							/* translators: %s expands to an invalid URL. */
							__( '%s does not seem to be a valid url. Please correct.', 'wordpress-seo' ),
							'<strong>' . esc_html( $submitted_url ) . '</strong>'
						),
						// CSS class for the WP notice.
						'notice-error'
					);
				}

				// Restore the previous URL value, if any.
				if ( isset( $old[ $key ] ) && $old[ $key ] !== '' ) {
					$url = WPSEO_Utils::sanitize_url( $old[ $key ] );
					if ( $url !== '' ) {
						$clean[ $key ] = $url;
					}
				}

				Yoast_Input_Validation::add_dirty_value_to_settings_errors( $key, $submitted_url );

				return;
			}

			// The URL format is valid, let's sanitize it.
			$url = WPSEO_Utils::sanitize_url( $validated_url );

			if ( $url !== '' ) {
				$clean[ $key ] = $url;
			}
		}
	}

	/**
	 * Validates a Facebook App ID.
	 *
	 * @param string $key   Key to check, in this case: the Facebook App ID field name.
	 * @param array  $dirty Dirty data with the new values.
	 * @param array  $old   Old data.
	 * @param array  $clean Clean data by reference, normally the default values.
	 */
	public function validate_facebook_app_id( $key, $dirty, $old, &$clean ) {
		if ( isset( $dirty[ $key ] ) && $dirty[ $key ] !== '' ) {
			$url = 'https://graph.facebook.com/' . $dirty[ $key ];

			$response = wp_remote_get( $url );
			// These filters are used in the tests.
			/**
			 * Filter: 'validate_facebook_app_id_api_response_code' - Allows to filter the Faceboook API response code.
			 *
			 * @api int $response_code The Facebook API response header code.
			 */
			$response_code = apply_filters( 'validate_facebook_app_id_api_response_code', wp_remote_retrieve_response_code( $response ) );
			/**
			 * Filter: 'validate_facebook_app_id_api_response_body' - Allows to filter the Faceboook API response body.
			 *
			 * @api string $response_body The Facebook API JSON response body.
			 */
			$response_body   = apply_filters( 'validate_facebook_app_id_api_response_body', wp_remote_retrieve_body( $response ) );
			$response_object = json_decode( $response_body );

			/*
			 * When the request is successful the response code will be 200 and
			 * the response object will contain an `id` property.
			 */
			if ( $response_code === 200 && isset( $response_object->id ) ) {
				$clean[ $key ] = $dirty[ $key ];
				return;
			}

			// Restore the previous value, if any.
			if ( isset( $old[ $key ] ) && $old[ $key ] !== '' ) {
				$clean[ $key ] = $old[ $key ];
			}

			if ( function_exists( 'add_settings_error' ) ) {
				add_settings_error(
					$this->group_name, // Slug title of the setting.
					$key, // Suffix-ID for the error message box. WordPress prepends `setting-error-`.
					sprintf(
						/* translators: %s expands to an invalid Facebook App ID. */
						__( '%s does not seem to be a valid Facebook App ID. Please correct.', 'wordpress-seo' ),
						'<strong>' . esc_html( $dirty[ $key ] ) . '</strong>'
					), // The error message.
					'notice-error' // CSS class for the WP notice, either the legacy 'error' / 'updated' or the new `notice-*` ones.
				);
			}

			Yoast_Input_Validation::add_dirty_value_to_settings_errors( $key, $dirty[ $key ] );
		}
	}

	/**
	 * Remove the default filters.
	 * Called from the validate() method to prevent failure to add new options.
	 *
	 * @return void
	 */
	public function remove_default_filters() {
		remove_filter( 'default_option_' . $this->option_name, array( $this, 'get_defaults' ) );
	}

	/**
	 * Get the enriched default value for an option.
	 *
	 * Checks if the concrete class contains an enrich_defaults() method and if so, runs it.
	 *
	 * {@internal The enrich_defaults method is used to set defaults for variable array keys
	 *            in an option, such as array keys depending on post_types and/or taxonomies.}}
	 *
	 * @return array
	 */
	public function get_defaults() {
		if ( method_exists( $this, 'translate_defaults' ) ) {
			$this->translate_defaults();
		}

		if ( method_exists( $this, 'enrich_defaults' ) ) {
			$this->enrich_defaults();
		}

		return apply_filters( 'wpseo_defaults', $this->defaults, $this->option_name );
	}

	/**
	 * Add filters to make sure that the option is merged with its defaults before being returned.
	 *
	 * @return void
	 */
	public function add_option_filters() {
		// Don't change, needs to check for false as could return prio 0 which would evaluate to false.
		if ( has_filter( 'option_' . $this->option_name, array( $this, 'get_option' ) ) === false ) {
			add_filter( 'option_' . $this->option_name, array( $this, 'get_option' ) );
		}
	}

	/**
	 * Remove the option filters.
	 * Called from the clean_up methods to make sure we retrieve the original old option.
	 *
	 * @return void
	 */
	public function remove_option_filters() {
		remove_filter( 'option_' . $this->option_name, array( $this, 'get_option' ) );
	}

	/**
	 * Merge an option with its default values.
	 *
	 * This method should *not* be called directly!!! It is only meant to filter the get_option() results.
	 *
	 * @param mixed $options Option value.
	 *
	 * @return mixed Option merged with the defaults for that option.
	 */
	public function get_option( $options = null ) {
		$filtered = $this->array_filter_merge( $options );

		/*
		 * If the option contains variable option keys, make sure we don't remove those settings
		 * - even if the defaults are not complete yet.
		 * Unfortunately this means we also won't be removing the settings for post types or taxonomies
		 * which are no longer in the WP install, but rather that than the other way around.
		 */
		if ( isset( $this->variable_array_key_patterns ) ) {
			$filtered = $this->retain_variable_keys( $options, $filtered );
		}

		return $filtered;
	}

	/* *********** METHODS influencing add_uption(), update_option() and saving from admin pages. *********** */

	/**
	 * Register (whitelist) the option for the configuration pages.
	 * The validation callback is already registered separately on the sanitize_option hook,
	 * so no need to double register.
	 *
	 * @return void
	 */
	public function register_setting() {
		if ( ! WPSEO_Capability_Utils::current_user_can( 'wpseo_manage_options' ) ) {
			return;
		}

		if ( $this->multisite_only === true ) {
			$network_settings_api = Yoast_Network_Settings_API::get();
			if ( $network_settings_api->meets_requirements() ) {
				$network_settings_api->register_setting( $this->group_name, $this->option_name );
			}
			return;
		}

		register_setting( $this->group_name, $this->option_name );
	}

	/**
	 * Validate the option
	 *
	 * @param mixed $option_value The unvalidated new value for the option.
	 *
	 * @return array Validated new value for the option.
	 */
	public function validate( $option_value ) {
		$clean = $this->get_defaults();

		/* Return the defaults if the new value is empty. */
		if ( ! is_array( $option_value ) || $option_value === array() ) {
			return $clean;
		}

		$option_value = array_map( array( 'WPSEO_Utils', 'trim_recursive' ), $option_value );

		$old = $this->get_original_option();
		if ( ! is_array( $old ) ) {
			$old = array();
		}
		$old = array_merge( $clean, $old );

		$clean = $this->validate_option( $option_value, $clean, $old );

		// Prevent updates to variables that are disabled via the override option.
		$clean = $this->prevent_disabled_options_update( $clean, $old );

		/* Retain the values for variable array keys even when the post type/taxonomy is not yet registered. */
		if ( isset( $this->variable_array_key_patterns ) ) {
			$clean = $this->retain_variable_keys( $option_value, $clean );
		}

		$this->remove_default_filters();

		return $clean;
	}

	/**
	 * Checks whether a specific option key is disabled.
	 *
	 * This is determined by whether an override option is available with a key that equals the given key prefixed
	 * with 'allow_'.
	 *
	 * @param string $key Option key.
	 *
	 * @return bool True if option key is disabled, false otherwise.
	 */
	public function is_disabled( $key ) {
		$override_option = $this->get_override_option();
		if ( empty( $override_option ) ) {
			return false;
		}

		return isset( $override_option[ self::ALLOW_KEY_PREFIX . $key ] ) && ! $override_option[ self::ALLOW_KEY_PREFIX . $key ];
	}

	/**
	 * All concrete classes must contain a validate_option() method which validates all
	 * values within the option.
	 *
	 * @param array $dirty New value for the option.
	 * @param array $clean Clean value for the option, normally the defaults.
	 * @param array $old   Old value of the option.
	 */
	abstract protected function validate_option( $dirty, $clean, $old );

	/* *********** METHODS for ADDING/UPDATING/UPGRADING the option. *********** */

	/**
	 * Retrieve the real old value (unmerged with defaults).
	 *
	 * @return array|bool The original option value (which can be false if the option doesn't exist).
	 */
	protected function get_original_option() {
		$this->remove_default_filters();
		$this->remove_option_filters();

		// Get (unvalidated) array, NOT merged with defaults.
		if ( $this->multisite_only !== true ) {
			$option_value = get_option( $this->option_name );
		}
		else {
			$option_value = get_site_option( $this->option_name );
		}

		$this->add_option_filters();
		$this->add_default_filters();

		return $option_value;
	}

	/**
	 * Add the option if it doesn't exist for some strange reason.
	 *
	 * @uses WPSEO_Option::get_original_option()
	 *
	 * @return void
	 */
	public function maybe_add_option() {
		if ( $this->get_original_option() === false ) {
			if ( $this->multisite_only !== true ) {
				update_option( $this->option_name, $this->get_defaults() );
			}
			else {
				$this->update_site_option( $this->get_defaults() );
			}
		}
	}

	/**
	 * Update a site_option.
	 *
	 * {@internal This special method is only needed for multisite options, but very needed indeed there.
	 *            The order in which certain functions and hooks are run is different between
	 *            get_option() and get_site_option() which means in practice that the removing
	 *            of the default filters would be done too late and the re-adding of the default
	 *            filters might not be done at all.
	 *            Aka: use the WPSEO_Options::update_site_option() method (which calls this method)
	 *            for safely adding/updating multisite options.}}
	 *
	 * @param mixed $value The new value for the option.
	 *
	 * @return bool Whether the update was succesfull.
	 */
	public function update_site_option( $value ) {
		if ( $this->multisite_only === true && is_multisite() ) {
			$this->remove_default_filters();
			$result = update_site_option( $this->option_name, $value );
			$this->add_default_filters();

			return $result;
		}
		else {
			return false;
		}
	}

	/**
	 * Retrieve the real old value (unmerged with defaults), clean and re-save the option.
	 *
	 * @uses WPSEO_Option::get_original_option()
	 * @uses WPSEO_Option::import()
	 *
	 * @param string $current_version Optional. Version from which to upgrade, if not set,
	 *                                version specific upgrades will be disregarded.
	 *
	 * @return void
	 */
	public function clean( $current_version = null ) {
		$option_value = $this->get_original_option();
		$this->import( $option_value, $current_version );
	}

	/**
	 * Clean and re-save the option.
	 *
	 * @uses clean_option() method from concrete class if it exists.
	 *
	 * @todo [JRF/whomever] Figure out a way to show settings error during/after the upgrade - maybe
	 * something along the lines of:
	 * -> add them to a property in this class
	 * -> if that property isset at the end of the routine and add_settings_error function does not exist,
	 *    save as transient (or update the transient if one already exists)
	 * -> next time an admin is in the WP back-end, show the errors and delete the transient or only delete it
	 *    once the admin has dismissed the message (add ajax function)
	 * Important: all validation routines which add_settings_errors would need to be changed for this to work
	 *
	 * @param array  $option_value          Option value to be imported.
	 * @param string $current_version       Optional. Version from which to upgrade, if not set,
	 *                                      version specific upgrades will be disregarded.
	 * @param array  $all_old_option_values Optional. Only used when importing old options to
	 *                                      have access to the real old values, in contrast to
	 *                                      the saved ones.
	 *
	 * @return void
	 */
	public function import( $option_value, $current_version = null, $all_old_option_values = null ) {
		if ( $option_value === false ) {
			$option_value = $this->get_defaults();
		}
		elseif ( is_array( $option_value ) && method_exists( $this, 'clean_option' ) ) {
			$option_value = $this->clean_option( $option_value, $current_version, $all_old_option_values );
		}

		/*
		 * Save the cleaned value - validation will take care of cleaning out array keys which
		 * should no longer be there.
		 */
		if ( $this->multisite_only !== true ) {
			update_option( $this->option_name, $option_value );
		}
		else {
			$this->update_site_option( $this->option_name, $option_value );
		}
	}

	/**
	 * Returns the variable array key patterns for an options class.
	 *
	 * @return array
	 */
	public function get_patterns() {
		return (array) $this->variable_array_key_patterns;
	}

	/**
	 * Concrete classes *may* contain a clean_option method which will clean out old/renamed
	 * values within the option.
	 */
	// abstract public function clean_option( $option_value, $current_version = null, $all_old_option_values = null );
	/* *********** HELPER METHODS for internal use. *********** */

	/**
	 * Helper method - Combines a fixed array of default values with an options array
	 * while filtering out any keys which are not in the defaults array.
	 *
	 * @todo [JRF] - shouldn't this be a straight array merge ? at the end of the day, the validation
	 * removes any invalid keys on save.
	 *
	 * @param array $options Optional. Current options. If not set, the option defaults
	 *                       for the $option_key will be returned.
	 *
	 * @return array Combined and filtered options array.
	 */
	protected function array_filter_merge( $options = null ) {

		$defaults = $this->get_defaults();

		if ( ! isset( $options ) || $options === false || $options === array() ) {
			return $defaults;
		}

		$options = (array) $options;

		/*
			$filtered = array();

			if ( $defaults !== array() ) {
				foreach ( $defaults as $key => $default_value ) {
					// @todo should this walk through array subkeys ?
					$filtered[ $key ] = ( isset( $options[ $key ] ) ? $options[ $key ] : $default_value );
				}
			}
		*/
		$filtered = array_merge( $defaults, $options );

		return $filtered;
	}

	/**
	 * Sets updated values for variables that are disabled via the override option back to their previous values.
	 *
	 * @param array $updated Updated option value.
	 * @param array $old     Old option value.
	 *
	 * @return array Updated option value, with all disabled variables set to their old values.
	 */
	protected function prevent_disabled_options_update( $updated, $old ) {
		$override_option = $this->get_override_option();
		if ( empty( $override_option ) ) {
			return $updated;
		}

		/*
		 * This loop could as well call `is_disabled( $key )` for each iteration,
		 * however this would be worse performance-wise.
		 */
		foreach ( $old as $key => $value ) {
			if ( isset( $override_option[ self::ALLOW_KEY_PREFIX . $key ] ) && ! $override_option[ self::ALLOW_KEY_PREFIX . $key ] ) {
				$updated[ $key ] = $old[ $key ];
			}
		}

		return $updated;
	}

	/**
	 * Retrieves the value of the override option, if available.
	 *
	 * An override option contains values that may determine access to certain sub-variables
	 * of this option.
	 *
	 * Only regular options in multisite can have override options, which in that case
	 * would be network options.
	 *
	 * @return array Override option value, or empty array if unavailable.
	 */
	protected function get_override_option() {
		if ( empty( $this->override_option_name ) || $this->multisite_only === true || ! is_multisite() ) {
			return array();
		}

		return get_site_option( $this->override_option_name, array() );
	}

	/**
	 * Make sure that any set option values relating to post_types and/or taxonomies are retained,
	 * even when that post_type or taxonomy may not yet have been registered.
	 *
	 * {@internal The wpseo_titles concrete class overrules this method. Make sure that any
	 *            changes applied here, also get ported to that version.}}
	 *
	 * @param array $dirty Original option as retrieved from the database.
	 * @param array $clean Filtered option where any options which shouldn't be in our option
	 *                     have already been removed and any options which weren't set
	 *                     have been set to their defaults.
	 *
	 * @return array
	 */
	protected function retain_variable_keys( $dirty, $clean ) {
		if ( ( is_array( $this->variable_array_key_patterns ) && $this->variable_array_key_patterns !== array() ) && ( is_array( $dirty ) && $dirty !== array() ) ) {
			foreach ( $dirty as $key => $value ) {

				// Do nothing if already in filtered options.
				if ( isset( $clean[ $key ] ) ) {
					continue;
				}

				foreach ( $this->variable_array_key_patterns as $pattern ) {

					if ( strpos( $key, $pattern ) === 0 ) {
						$clean[ $key ] = $value;
						break;
					}
				}
			}
		}

		return $clean;
	}

	/**
	 * Check whether a given array key conforms to one of the variable array key patterns for this option.
	 *
	 * @usedby validate_option() methods for options with variable array keys.
	 *
	 * @param string $key Array key to check.
	 *
	 * @return string Pattern if it conforms, original array key if it doesn't or if the option
	 *                does not have variable array keys.
	 */
	protected function get_switch_key( $key ) {
		if ( ! isset( $this->variable_array_key_patterns ) || ( ! is_array( $this->variable_array_key_patterns ) || $this->variable_array_key_patterns === array() ) ) {
			return $key;
		}

		foreach ( $this->variable_array_key_patterns as $pattern ) {
			if ( strpos( $key, $pattern ) === 0 ) {
				return $pattern;
			}
		}

		return $key;
	}
}