3v4l.org

run code in 300+ PHP versions simultaneously
<?php $from = range(1, 25000); $to = array_combine(range(25001, 50000), range(25001, 50000)); foreach($from as $i => $v) { $to = array_merge($from, array($i => $v)); echo $i."\n"; }

preferences:
32.78 ms | 402 KiB | 5 Q