$array = array('header' => array('3457' => Array('0' => 'test'), '3458' => Array('0' => 'kitap test') ));

$ids = array();
array_walk($array['header'], function ($value, $key) use ( &$ids ) {
if (strpos($value[0], 'test') !== false) {
        $ids[] = $key;
    }
});

var_dump( $ids );