3v4l.org

run code in 300+ PHP versions simultaneously
<?php $a = "aaaaaaaaaaaaaaaababababababababa"; while(strpos($a, "aa")!==false ||strpos($a, "ba")!==false) { $a = str_replace(array("aa", "ba"), "a", $a); } echo $a;

preferences:
66.53 ms | 402 KiB | 5 Q