PHPUnit:我如何模拟这个文件系统?

Consider the following scenario (this is not production code):

 class MyClass {
    public function myMethod() {
        // create a directory
        $path = sys_get_temp_dir() . '/' . md5(rand());
        if(!mkdir($path)) {
            throw new Exception("mkdir() failed.");
        }

        // create a file in that folder
        $myFile = fopen("$path/myFile.txt", "w");
        if(!$myFile) {
            throw new Exception("Cannot open file handle.");
        }
    }
}

好吧，那有什么问题呢?代码覆盖率报告此行未被覆盖:

throw new Exception("Cannot open file handle.");