- print_r: documentation ( source)
- array_keys: documentation ( source)
<?php
$array[0]['d'] = 'd1';
$array[0]['a'] = 'a1';
$array[1]['d'] = 'd2';
$array[1]['a'] = 'a2';
$array[2]['d'] = 'd3';
$array[2]['a'] = 'a3';
$array[3]['d'] = 'd1';
$array[3]['a'] = 'a1';
$keys = array_keys($array,"d1");
print_r($keys);
?>