set_transient

 *
 * If the transient does not exist, does not have a value, or has expired,
 * then the return value will be false.
 *
 * @since 2.8.0
 *
 * @param string $transient Transient name. Expected to not be SQL-escaped.
 * @return mixed Value of transient.
 */
function get_transient( $transient ) {

	/**
	 * Filters the value of an existing transient before it is retrieved.
	 *
	 * The dynamic portion of the hook name, `$transient`, refers to the transient name.
	 *
	 * Returning a truthy value from the filter will effectively short-circuit retrieval
	 * and return the passed value instead.
	 *
	 * @since 2.8.0
	 * @since 4.4.0 The `$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 that 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, `$transient`, refers to the transient name.
	 *
	 * @since 2.8.0
	 * @since 4.4.0 The `$transient` parameter was added
	 *
	 * @param mixed  $value     Value of transient.
	 * @param string $transient Transient name.
	 */
	return apply_filters( "transient_{$transient}", $value, $transient );
}

/**
 * Sets/updates the value of a transient.
 *
 * You do not need to serialize values. If the value needs to be serialized,
 * then it will be serialized before it is set.
 *
 * @since 2.8.0
 *
 * @param string $transient  Transient name. Expected to not be SQL-escaped.
 *                           Must be 172 characters or fewer in length.
 * @param mixed  $value      Transient value. Must be serializable if non-scalar.
 *                           Expected to not be SQL-escaped.
 * @param int    $expiration Optional. Time until expiration in seconds. Default 0 (no expiration).
 * @return bool True if the value was set, false otherwise.
 */
function set_transient( $transient, $value, $expiration = 0 ) {

	$expiration = (int) $expiration;

	/**
	 * Filters a specific transient before its value is set.
	 *
	 * The dynamic portion of the hook name, `$transient`, refers to the transient name.
	 *
	 * @since 3.0.0