- array_keys: documentation ( source)
<?php
$answers = array(
'answer' => array(
-1 => array(
-1 => 'first',
-2 => 'second',
),
-2 => array(
-3 => 'third'
),
),
'incorrect' => array(),
);
foreach (array_keys($answers['answer'][-1]) as $answer) {
echo $answer;
}