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run code in 300+ PHP versions simultaneously
<?php $arr = array('a', 'b', 'c', 'd', 'e', 'f'); $str = implode(array_walk(function($v,$k){if($k%3==0)$v.=';';}, array_reverse($arr))); echo $str;

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44.18 ms | 402 KiB | 5 Q