为啥无法输出一张图片

为什么无法输出一张图片

	session_start();


	//随机码的个数

	//$_rnd_code = 4;


	//创建随机码

	for($i=0;$i
		$_nmsg.=dechex(mt_rand(0,15));

	}


	//保存session

	$_SESSION['code']=$_nmsg;


	//长和高

	$_width = 75;

	$_height = 25;


	//创建一张图像

	$_img = imagecreatetruecolor($_width,$_height);




	//输出图像

	header('Content-Type:image/png');

	imagepng($_img);

输出结果为

Notice:  Undefined variable: _rnd_code in D:\AMP\web\ide_tmp.php on line 53



Warning:  Division by zero in D:\AMP\web\ide_tmp.php on line 53



Notice:  Undefined variable: _heght in D:\AMP\web\ide_tmp.php on line 53



Warning:  mt_rand(): max(0) is smaller than min(1) in D:\AMP\web\ide_tmp.php on line 53



Notice:  Undefined variable: _rnd_code in D:\AMP\web\ide_tmp.php on line 53



Warning:  Division by zero in D:\AMP\web\ide_tmp.php on line 53



Notice:  Undefined variable: _heght in D:\AMP\web\ide_tmp.php on line 53



Warning:  mt_rand(): max(0) is smaller than min(1) in D:\AMP\web\ide_tmp.php on line 53



Notice:  Undefined variable: _rnd_code in D:\AMP\web\ide_tmp.php on line 53



Warning:  Division by zero in D:\AMP\web\ide_tmp.php on line 53



Notice:  Undefined variable: _heght in D:\AMP\web\ide_tmp.php on line 53



Warning:  mt_rand(): max(0) is smaller than min(1) in D:\AMP\web\ide_tmp.php on line 53



Notice:  Undefined variable: _rnd_code in D:\AMP\web\ide_tmp.php on line 53



Warning:  Division by zero in D:\AMP\web\ide_tmp.php on line 53



Notice:  Undefined variable: _heght in D:\AMP\web\ide_tmp.php on line 53



Warning:  mt_rand(): max(0) is smaller than min(1) in D:\AMP\web\ide_tmp.php on line 53



Notice:  Undefined variable: _rnd_code in D:\AMP\web\ide_tmp.php on line 53



Warning:  Division by zero in D:\AMP\web\ide_tmp.php on line 53



Notice:  Undefined variable: _heght in D:\AMP\web\ide_tmp.php on line 53



Warning:  mt_rand(): max(0) is smaller than min(1) in D:\AMP\web\ide_tmp.php on line 53


‰PNG

按错误信息修改
然后得：
仍然是不友好的“验证码”图片
将header注释掉，打开错误就可以看到错误。

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