Find all second level keys in multi-dimensional array in php

array_keys(call_user_func_array('array_merge', $a));

Merge all values and retrieve the resulting keys.

<?php

// Gets a list of all the 2nd-level keys in the array
function getL2Keys($array)
{
    $result = array();
    foreach($array as $sub) {
        $result = array_merge($result, $sub);
    }        
    return array_keys($result);
}

?>

edit: removed superfluous array_reverse() function