跟学姐要的毕业设计运行不了

function _yanzhengma() {
$_width = 75;
$_height = 25;
$_rnd_code = 4;
//创建随机码
for ($i=0;$i<$_rnd_code;$i++) {
$_nmsg .= dechex(mt_rand(0,15));
}
//保存在session
$_SESSION['yanzhengma'] = $_nmsg;
//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);
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($_SESSION['yanzhengma']);$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,$_height/2),$_SESSION['yanzhengma'][$i],$_rnd_color);
}
//输出图像
header('Content-Type: image/png');
imagepng($_img);
//销毁
imagedestroy($_img);
}
这个验证码的代码哪里有问题吗