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<?php $start= "2013-01-31"; $current = date("Y-m-d", strtotime("1st", strtotime($start))); var_dump($current); ?>
Output for 5.1.0 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
string(10) "1970-01-01"
Output for 4.3.0 - 5.0.5
string(10) "2013-01-31"