如何使用PHP限制上传文件的大小？

Question

我想限制可以上传到远程服务器的图像的文件大小。我编写了以下代码，它允许我成功上传在我的图库中正常工作的文件。

但是，我想将文件大小限制为大约2 MB。此代码适用于上载文件，但不限制文件大小。此外，我认为PHP具有内置的最大上传文件大小。使用此代码，我甚至可以上传一个31.4MB .mp4文件。这是代码：

<?php 
$max = 2000 * 1024;
$message = '';
if (isset($_POST['upload'])) {
    $destination = __DIR__ . '/_images/';
    if ($_FILES['filename']['error'] == 0) {
        $result = move_uploaded_file($_FILES['filename']['tmp_name'], $destination 
            . $_FILES['filename']['name']);
        if ($result) {
            $message = $_FILES['filename']['name'] ;
        } else {
            $message = 'Sorry, there was a problem uploading ' 
                .$_FILES['filename']['name'];
        }
    } else {
        switch ($_FILES['filename']['error']) {
            case 2:
                $message = $_FILES['filename']['name'] . ' is too big to upload.';
                break;
            case 4:
                $message = 'No file selected.';
                break;
            default:
                $message = 'Sorry, there was a problem uploading ' 
                    .$_FILES['filename']['name'];
                break;
        }
    }
}
?>