- date: documentation ( source)
- strtotime: documentation ( source)
<?php
//Your starting date as DateTime
$currentDate = new DateTime(date('Y-m-d'));
//Add 1 month
$currentDate->add(new DateInterval('P1M'));
//Get the first day of the next month as string
$firstDayOfNextMonth = strtotime($currentDate->format('Y-m-1'));
echo date('m/d/Y', $firstDayOfNextMonth);