$arr = array(
       'key1' => array(1),
       'key2' => array(1),
       'key3' => array(1),
       'key4' => array(1),
       'key5' => array(6)
);

uasort($arr, function ($a, $b)
{
      if ($a == $b) {
                return 0;
      }
      return ($a < $b) ? -1 : 1;
});

Array
(
    [key4] => Array
        (
            [0] => 1
        )

    [key1] => Array
        (
            [0] => 1
        )

    [key3] => Array
        (
            [0] => 1
        )

    [key2] => Array
        (
            [0] => 1
        )

    [key5] => Array
        (
            [0] => 6
        )
)

 
uasort($arr, function ($a, $b)
{
            if ($a[0] == $b[0]) {
                return 0;
            }
            return ($a[0] < $b[0]) ? -1 : 1;
 });

'key1' => array(1),
       'key2' => array(1),
       'key3' => array(1),
       'key4' => array(1),