get_transient( string $transient )

function get_transient( $transient ) {

	/**
	 * Filters the value of an existing transient.
	 *
	 * The dynamic portion of the hook name, `$transient`, refers to the transient name.
	 *
	 * Passing a truthy value to the filter will effectively short-circuit retrieval
	 * of the transient, returning the passed value instead.
	 *
	 * @since WP-2.8.0
	 * @since WP-4.4.0 The `$transient` parameter was added
	 *
	 * @param mixed  $pre_transient The default value to return if the transient does not exist.
	 *                              Any value other than false will short-circuit the retrieval
	 *                              of the transient, and return the returned value.
	 * @param string $transient     Transient name.
	 */
	$pre = apply_filters( "pre_transient_{$transient}", false, $transient );
	if ( false !== $pre )
		return $pre;

	if ( wp_using_ext_object_cache() ) {
		$value = wp_cache_get( $transient, 'transient' );
	} else {
		$transient_option = '_transient_' . $transient;
		if ( ! wp_installing() ) {
			// If option is not in alloptions, it is not autoloaded and thus has a timeout
			$alloptions = wp_load_alloptions();
			if ( !isset( $alloptions[$transient_option] ) ) {
				$transient_timeout = '_transient_timeout_' . $transient;
				$timeout = get_option( $transient_timeout );
				if ( false !== $timeout && $timeout < time() ) {
					delete_option( $transient_option  );
					delete_option( $transient_timeout );
					$value = false;
				}
			}
		}

		if ( ! isset( $value ) )
			$value = get_option( $transient_option );
	}

	/**
	 * Filters an existing transient's value.
	 *
	 * The dynamic portion of the hook name, `$transient`, refers to the transient name.
	 *
	 * @since WP-2.8.0
	 * @since WP-4.4.0 The `$transient` parameter was added
	 *
	 * @param mixed  $value     Value of transient.
	 * @param string $transient Transient name.
	 */
	return apply_filters( "transient_{$transient}", $value, $transient );
}