3v4l.org

run code in 300+ PHP versions simultaneously
<?php $start= "2013-01-31"; $current = date("Y-m-d", strtotime("1st", strtotime($start))); var_dump($current); ?>
Output for 5.1.0 - 5.1.6, 5.2.0 - 5.2.17, 5.3.0 - 5.3.29, 5.4.0 - 5.4.45, 5.5.0 - 5.5.38, 5.6.0 - 5.6.28, 7.0.0 - 7.0.20, 7.1.0 - 7.1.25, 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.27, 8.2.0 - 8.2.17, 8.3.0 - 8.3.4
string(10) "1970-01-01"
Output for 4.3.0 - 4.3.11, 4.4.0 - 4.4.9, 5.0.0 - 5.0.5
string(10) "2013-01-31"

preferences:
274.95 ms | 402 KiB | 364 Q