PHP: Grouping Elements in Array by Specific Key Field

This is the code for grouping elements in array by specific key field of the element.

Code

<?php

  function groupBySpecificKey(array& $source, $key){
    $map = array();
    foreach($source as $elem){
      $groupKey = $elem[$key];
      if(is_null($groupKey)) continue;
      $map[$groupKey][] = $elem;
    }
    return $map;
  } 

Example

  $result = groupBySpecificKey($source, 'country');
  var_dump($result);

  $source = array(
     array('id' => 1, 'name' =>'Joe', 'country' => 'China'),
     array('id' => 2, 'name' =>'Chris', 'country' => 'USA'),
     array('id' => 3, 'name' =>'Tod', 'country' => 'USA'),
  );

The result is below.

array(2) {
  ["China"]=>
  array(1) {
    [0]=>
    array(3) {
      ["id"]=>
      int(1)
      ["name"]=>
      string(3) "Joe"
      ["country"]=>
      string(5) "China"
    }
  }
  ["USA"]=>
  array(2) {
    [0]=>
    array(3) {
      ["id"]=>
      int(2)
      ["name"]=>
      string(5) "Chris"
      ["country"]=>
      string(3) "USA"
    }
    [1]=>
    array(3) {
      ["id"]=>
      int(3)
      ["name"]=>
      string(4) "Tod"
      ["country"]=>
      string(3) "USA"
    }
  }
}