奇怪的PHP json_encode错误：json_encode（）最多需要2个参数，给定3个

This code:

$arr = array(
    'a' => 1,
    'b' => array(
        'c' => 'Hello',
        'd' => [1, 10, 100, 1001]
    ),
);

echo json_encode($arr, JSON_PRETTY_PRINT, 2);
$arr = array(
    'a' => 1,
    'b