为什么无法输出一张图片

[mxz159951]

	session_start();
	
	//随机码的个数
	//$_rnd_code = 4;
	
	//创建随机码
	for($i=0;$i<4;$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

[一起混吧]

$_nmsg=''; for($i=0;$i<4;$i++){ $_nmsg.=dechex(mt_rand(0,15)); }

[mxz159951]

引用 1 楼 jordan102 的回复:

$_nmsg='';
for($i=0;$i<4;$i++){
$_nmsg.=dechex(mt_rand(0,15));
}

	session_start();
	
	//随机码的个数
	//$_rnd_code = 4;
	
	$_nmsg=' ';
	//创建随机码
	for($i=0;$i<4;$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);

		//白色
	$_white = imagecolorallocate($_img,255,255,255);
	
	//填充
	imagefill($_img,0,0,$_white);
	
	$_flag = false;
	
	if($_flag){
	//黑色边框
	$_black = imagecolorallocate($_img,0,0,0);
	imagerectangle($_img,0,0,$_width-1,$_height-1,$_black);
	}
	
	//随机画出6个线条
	for($i=0;$i<6;$i++){
		$_rnd_color = imagecolorallocate($_img,mt_rand(0,255),mt_rand(0,255),mt_rand(0,255));
		imageline($_img,mt_rand(0,$_width),mt_rand(0,$_height),mt_rand(0,$_width),mt_rand(0,$_height),$_rnd_color);
		
	}
	
	//随机雪花
	for($i=0;$i<100;$i++){
		$_rnd_color = imagecolorallocate($_img,mt_rand(200,255),mt_rand(200,255),mt_rand(200,255));
		imagestring($_img,1,mt_rand(1,$_width),mt_rand(1,$_height),'*',$_rnd_color);
	}
	
	//输出验证码
	for($i=0;$i<strlen($_SESSION['code']);$i++){
		$_rnd_color = imagecolorallocate($_img,mt_rand(0,100),mt_rand(0,150),mt_rand(0,200));
		imagestring($_img,5,$i*$_width/$_rnd_code+mt_rand(1,10),mt_rand(1,$_heght/2),$_SESSION['code'][$i],$_rnd_color);
	}
	

	//输出图像
	header('Content-Type:image/png');
	imagepng($_img);

写完之后，还是显示

[xuzuning]

<br />
<b>Notice</b>:  Undefined variable: _rnd_code in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Warning</b>:  Division by zero in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Notice</b>:  Undefined variable: _heght in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Warning</b>:  mt_rand(): max(0) is smaller than min(1) in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Notice</b>:  Undefined variable: _rnd_code in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Warning</b>:  Division by zero in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Notice</b>:  Undefined variable: _heght in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Warning</b>:  mt_rand(): max(0) is smaller than min(1) in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Notice</b>:  Undefined variable: _rnd_code in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Warning</b>:  Division by zero in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Notice</b>:  Undefined variable: _heght in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Warning</b>:  mt_rand(): max(0) is smaller than min(1) in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Notice</b>:  Undefined variable: _rnd_code in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Warning</b>:  Division by zero in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Notice</b>:  Undefined variable: _heght in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Warning</b>:  mt_rand(): max(0) is smaller than min(1) in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Notice</b>:  Undefined variable: _rnd_code in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Warning</b>:  Division by zero in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Notice</b>:  Undefined variable: _heght in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
<br />
<b>Warning</b>:  mt_rand(): max(0) is smaller than min(1) in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br />
‰PNG

按错误信息修改
然后得：
仍然是不友好的“验证码”图片

[一起混吧]

将header注释掉，打开错误就可以看到错误。