- array_count_values: documentation ( source)
- print_r: documentation ( source)
- array_filter: documentation ( source)
- array_keys: documentation ( source)
<?php
$yourArr = [1,2,1,2,3,4,5];
$res = array_count_values($yourArr);
$res = array_filter($res, function($item){ return $item == 1; });
print_r(array_keys($res));