相同数组的组合为新数组

array (size=2)
  'id' => int 139
  'key' => string '11111' 

array (size=2)
  'id' => int 139
  'key' => string '22222' 

array (size=2)
  'id' => int 159
  'key' => string '33333'

array (size=2)
  'id' => int 159
  'key' => string '44444'

合并为

array (size=2)
  'id' => int 139
  'key' => string '11111，22222' 

array (size=2)
  'id' => int 159
  'key' => string '33333，44444'

---

$arr = [
    ["id"=>139, "key"=>"111"],
    ["id"=>139, "key"=>"222"],
    ["id"=>159, "key"=>"333"],
    ["id"=>159, "key"=>"444"]
];

$res = [];
foreach( $arr as $item ) $res[ $item['id'] ][] = $item['key'];