why doesn't this php foreach return the value?

why doesn't this php foreach return the value?

If you look at the code below, it doesn't echo anything out when the
return is in the if statement. When I take the return out, it echoes out
the correct value. Why is this?
$images= Array(
[0] => Array
(
[id] => 131],
[width] => 400]
),
[1] => Array
(
[id] => 140],
[width] => 900]
)
)
$array_key=0;
foreach($images as $key => $image){
if($image['id'] == $image_id){
$array_key= $key;
return;
}
}
echo $array_key;