為啥無法輸出一張圖片

為什麼無法輸出一張圖片

session_start();

//隨機碼的個數
//$_rnd_code = 4;

//建立隨機碼
for($i=0;$i<4;$i++){
$_nmsg.=dechex(mt_rand(0,15));
}

//儲存session
$_SESSION['code']=$_nmsg;

//長和高
$_width = 75;
$_height = 25;

//建立一張映像
$_img = imagecreatetruecolor($_width,$_height);



//輸出映像
header('Content-Type:image/png');
imagepng($_img);

輸出結果為
------解決方案--------------------

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

按錯誤資訊修改
然後得：
仍然是不友好的“驗證碼”圖片

------解決方案--------------------
將header注釋掉，開啟錯誤就可以看到錯誤。

