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run code in 300+ PHP versions simultaneously
<?php $string = 'abc'; var_dump($string, preg_replace_callback( '/(.)/', function ($m) { static $counter = 1; return $m[1] . $counter++; }, $string ));
Output for 7.1.0 - 7.1.33, 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(3) "abc" string(6) "a1b2c3"

preferences:
184.13 ms | 403 KiB | 185 Q