- var_dump: documentation ( source)
- array_search: documentation ( source)
- print_r: documentation ( source)
<?php
$array = array(0 => 'blue', 1 => 'red', 2 => 'green', 3 => 'red');
$key = array_search('green', $array); // $key = 2;
$key = array_search('yellow', $array); // $key = 1;
print_r(var_dump($key));