画像をエクスポートできないのはなぜですか?

	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);

	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);

<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