$date1=new DateTime('2018-04-23'); // date object created.
$date1->add(new DateInterval('P1Y3M')); // interval of 1 year 3 months added
echo $date1->format('Y-M-d'); // Output is 2019-Jul-23
/// by using DateTimeImmutable ///
$date1=new DateTimeImmutable('2018-04-23'); // date object created .
$date1->add(new DateInterval('P1Y3M')); // interval of 1 year 3 months added
echo $date1->format('Y-M-d'); // Output is 2018-Apr-23