PHP sort multi-dimension array by value

Below is the example code for sorting multi-dimension array by its value.

$my_array = array (

  '0' => array (
      'ID' => 425,
      'ln' => 'CBolware',
      'fn' => 'Christian',
      'mi' => '',
     ),
  '1' => array (
      'ID' => 423,
      'ln' => 'Bernstein',
      'fn' => 'ZBear',
      'mi' => 'D.',
     ),
  '2' => array (
     'ID' => 419,
     'ln' => 'DBellweather',
     'fn' => 'Brent',
     'mi' => '',
    ),
  '3' => array (
     'ID' => 356,
     'ln' => 'ABayleaf, III',
     'fn' => 'Joe',
     'mi' => 'X.',
    ),
  '4 '=> array (
     'ID' => 336,
     'ln' => 'Public',
     'fn' => 'John',
     'mi' => 'Q.',
    ),
);

usort($my_array, function($a, $b) {
   return strcmp($a['fn'], $b['fn']);  // fn -> key of the array
});

print_r($my_array);