您要删除数组的第四个值，而不是该值.改用 array_search

You're removing the fourth value of the array, not the value. Use array_search instead

$filters = '{"1":1,"2":2,"3":4}';
$fobj = json_decode($filters, TRUE);
$search = array_search(4, $fobj);
if($search !== false) unset($fobj[$search]);
echo $filters = json_encode($fobj );