php实现汉字验证码和算式验证码的方法

<?php
session_start();
header("Content-type: image/png");
$num1 = mt_rand(0,9);//第一位数
$num2 = mt_rand(1,9);//第二位数
$type_str = "+-*";//办法字符串集合
$type = substr($type_str,rand(0,2),1);//随机办法
$change = mt_rand(1,3);
if($change==1){
?$code = "$num1$type$num2=?";
?$result = "$verifyCode=$num1$type$num2;";
?eval($result);
?$_SESSION['authnum_session'] = $verifyCode;?
}elseif($change==2){
?$result = "$verifyCode=$num1$type$num2;";
?eval($result);
??? $code = $num1.$type."_=".$verifyCode;
?$_SESSION['authnum_session'] = $num2;?
}elseif($change==3){
?$result = "$verifyCode=$num1$type$num2;";
?eval($result);
??? $code = "_".$type.$num2."=".$verifyCode;
?$_SESSION['authnum_session'] = $num1;?
}
$im = imagecreate(68,28);??
$black = imagecolorallocate($im,0);????
$white = imagecolorallocate($im,255,255);
$gray = imagecolorallocate($im,200,200);
$red = imagecolorallocate($im,0);
imagefill($im,$white);???? ??
imagestring($im,5,10,8,$code,$black);???
for($i=0;$i<70;$i++) {
?imagesetpixel($im,mt_rand(0,58),28),$black);
?imagesetpixel($im,$red);
?imagesetpixel($im,$gray);
}
imagepng($im);
imagedestroy($im);
?>