PHP 连续上传超过 4 个文件时出错

当我尝试上传四个或少于四个文件时，它工作正常。但是当我尝试上传四个以上的文件时，出现以下错误：

Warning: Invalid argument supplied for foreach() in C:'wamp'www'multiple-file-upload-with-php'index.php on line 16
Notice: Undefined index: files in C:'wamp'www'multiple-file-upload-with-php'index.php on line 16

我正在使用以下 HTML：

    <!-- Multiple file upload html form-->
    <form action="" method="post" enctype="multipart/form-data">
        <input type="file" name="files[]" multiple="multiple" accept="image/*">
        <input type="submit" value="Upload">
    </form>

和 PHP：

if(isset($_POST) and $_SERVER['REQUEST_METHOD'] == "POST"){
$valid_formats = array("jpg", "png", "gif", "zip", "bmp");
$max_file_size = 1024*100; //100 kb
$path = "uploads/"; // Upload directory
$count = 0;
// Loop $_FILES to execute all files
foreach ($_FILES['files']['name'] as $f => $name) {     
    if ($_FILES['files']['error'][$f] == 4) {
        continue; // Skip file if any error found
    }          
    if ($_FILES['files']['error'][$f] == 0) {              
        if ($_FILES['files']['size'][$f] > $max_file_size) {
            $message[] = "$name is too large!.";
            continue; // Skip large files
        }
        elseif( ! in_array(pathinfo($name, PATHINFO_EXTENSION), $valid_formats) ){
            $message[] = "$name is not a valid format";
            continue; // Skip invalid file formats
        }
        else{ // No error found! Move uploaded files 
            if(move_uploaded_file($_FILES["files"]["tmp_name"][$f], $path.$name)) {
                $count++; // Number of successfully uploaded files
            }
        }
    }
}
}

可能有什么问题？