Comment by program

Also arrays of arrays have a much more compact syntax:

  $a = [1 => [4, 5, 6], 2 => [7, 8, 9], 3 => [10, 11, 12]];

  $a = array(1 => array(4, 5, 6), 2 => array(7, 8, 9), 3 => array(10, 11, 12));