- in_array: documentation ( source)
<?php
$array = array();
$array[2] = (string)"001812000000000000000000000000000011006";
$array[3] = (string)"001812501701000000000000000000000026003";
if(in_array((string)"001812501701000000000000000000000006012", $array))echo 'ist drinn';