Sort a PHP array by another array order and date

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This solution works fine in php 5.3+, if you are trapped in an older version you can easily replace DateTime objects with calls to strptime.

Products with a category that has no position specified are pushed to the end of the list (see product with cat == 42 in the example).

$cats = array(1,6,3,4,7,2);

// added a few products to test the date sorting
$prods = array(
  array('cat'=>1,'date'=>'3/3/2011'),
  array('cat'=>1,'date'=>'2/3/2011'),
  array('cat'=>1,'date'=>'1/3/2011'),
  array('cat'=>42,'date'=>'2/3/2011'),
  array('cat'=>2,'date'=>'1/3/2011'),
  array('cat'=>2,'date'=>'2/3/2011'),
  array('cat'=>2,'date'=>'1/6/2011'),
  array('cat'=>3,'date'=>'2/3/2011')
);

// need an index of category_id => position wanted in the sort
$r_cats = array_flip($cats);
// if the category of a product is not found in the requested sort, we will use 0, and product with category id 0 will be sent to the end of the sort
$r_cats[0] = count($r_cats);
// this one is needed for DateTime, put whatever your timezone is (not important for the use we do of it, but required to avoid a warning)
date_default_timezone_set('Europe/Paris');

usort($prods, function($a, $b) use($r_cats) {
  $cat_a = isset($r_cats[$a['cat']]) ? $r_cats[$a['cat']] : $r_cats[0];
  $cat_b = isset($r_cats[$b['cat']]) ? $r_cats[$b['cat']] : $r_cats[0];

  if ($cat_a < $cat_b) {
    return -1;
  } elseif ($cat_a > $cat_b) {
    return 1;
  }

  $date_a = DateTime::createFromFormat('j/n/Y', $a['date']);
  $date_b = DateTime::createFromFormat('j/n/Y', $b['date']);

  if ($date_a < $date_b) {
    return -1;
  } elseif ($date_a > $date_b) {
    return 1;
  }

  return 0;
});

var_dump($prods);

The results are as follow:

array(8) {
  [0]=>
  array(2) {
    ["cat"]=>
    int(1)
    ["date"]=>
    string(8) "1/3/2011"
  }
  [1]=>
  array(2) {
    ["cat"]=>
    int(1)
    ["date"]=>
    string(8) "2/3/2011"
  }
  [2]=>
  array(2) {
    ["cat"]=>
    int(1)
    ["date"]=>
    string(8) "3/3/2011"
  }
  [3]=>
  array(2) {
    ["cat"]=>
    int(3)
    ["date"]=>
    string(8) "2/3/2011"
  }
  [4]=>
  array(2) {
    ["cat"]=>
    int(2)
    ["date"]=>
    string(8) "1/3/2011"
  }
  [5]=>
  array(2) {
    ["cat"]=>
    int(2)
    ["date"]=>
    string(8) "2/3/2011"
  }
  [6]=>
  array(2) {
    ["cat"]=>
    int(2)
    ["date"]=>
    string(8) "1/6/2011"
  }
  [7]=>
  array(2) {
    ["cat"]=>
    int(42)
    ["date"]=>
    string(8) "2/3/2011"
  }
}