$ocenka = Array
(
 [4] => "7",
 [5617] => "4",
 [1] => "2",
 [222] => "1",
 [2748] => "2",
 [2643] => "3"
);

$coment = Array

(
 [4] => "2",
 [11] => "21"
);

Должно получиться

общ:
Array
(
 [4] => 9
 [5617] => 4
 [1] => 2
 [222] => 1
 [2748] => 2
 [2643] => 3
 [11] => 21
)
Решение:
$ocenka = Array
(
 [4] => "7",
 [5617] => "4",
 [1] => "2",
 [222] => "1",
 [2748] => "2",
 [2643] => "3"
);
$coment = Array
(
 [4] => "2",
 [11] => "21"
);
<?
foreach($ocenka as $key => $val)
{
$obs[$key]=$coment[$key] + $val;
unset($coment[$key]);
}
 foreach($coment as $key2 => $val2)
 {
 $obs[$key2]=$obs[$key2]+$val2;
 }
print_r($obs);
?>