Correctly determine if date string is a valid date in that format

Question

I\'m receiving a date string from an API, and it is formatted as yyyy-mm-dd.

I am currently using a regex to validate the string format, which works ok,

Answer 1

    /**** date check is a recursive function. it's need 3 argument 
    MONTH,DAY,YEAR. ******/

    $always_valid_date = $this->date_check($month,$day,$year);

    private function date_check($month,$day,$year){

        /** checkdate() is a php function that check a date is valid 
        or not. if valid date it's return true else false.   **/

        $status = checkdate($month,$day,$year);

        if($status == true){

            $always_valid_date = $year . '-' . $month . '-' . $day;

            return $always_valid_date;

        }else{
            $day = ($day - 1);

            /**recursive call**/

            return $this->date_check($month,$day,$year);
        }

    }