画像をエクスポートできないのはなぜですか?

写真をエクスポートできないのはなぜですか

	session_start();<br />
	<br />
	//随机码的个数<br />
	//$_rnd_code = 4;<br />
	<br />
	//创建随机码<br />
	for($i=0;$i<4;$i++){<br />
		$_nmsg.=dechex(mt_rand(0,15));<br />
	}<br />
	<br />
	//保存session<br />
	$_SESSION['code']=$_nmsg;<br />
	<br />
	//长和高<br />
	$_width = 75;<br />
	$_height = 25;<br />
	<br />
	//创建一张图像<br />
	$_img = imagecreatetruecolor($_width,$_height);<br />
<br />
	<br />
<br />
	//输出图像<br />
	header('Content-Type:image/png');<br />
	imagepng($_img);

出力結果は

<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 />
<br /><br />
<b>Warning</b>:  Division by zero in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br /><br />
<br /><br />
<b>Notice</b>:  Undefined variable: _heght in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br /><br />
<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 />
<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 />
<br /><br />
<b>Warning</b>:  Division by zero in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br /><br />
<br /><br />
<b>Notice</b>:  Undefined variable: _heght in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br /><br />
<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 />
<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 />
<br /><br />
<b>Warning</b>:  Division by zero in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br /><br />
<br /><br />
<b>Notice</b>:  Undefined variable: _heght in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br /><br />
<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 />
<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 />
<br /><br />
<b>Warning</b>:  Division by zero in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br /><br />
<br /><br />
<b>Notice</b>:  Undefined variable: _heght in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br /><br />
<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 />
<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 />
<br /><br />
<b>Warning</b>:  Division by zero in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br /><br />
<br /><br />
<b>Notice</b>:  Undefined variable: _heght in <b>D:\AMP\web\ide_tmp.php</b> on line <b>53</b><br /><br />
<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 />
‰PNG<br />


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を取得します。 まだ不親切な「確認コード」の図です
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