为何crontab -e不能执行fwrite写入文件？蹦溃了

为什么crontab -e不能执行fwrite写入文件？蹦溃了。

function Write_FileFun_new($filename,$filedir, $htmldata = '') {
	if(!is_dir($filedir)) {
		mkdir($filedir, 0777);
	}
	$htmlfile = $filedir.$filename;
	if($fp = fopen($htmlfile, 'wbt')) {
		fwrite($fp,$htmldata);
		fclose($fp);
	} else {
		echo "
Can not write to files, please check directory $htmldir.";
		exit;
	}
}
$testnum_temp="";
Write_FileFun_new("test.php","inc/", $testnum_temp);