利用php自身带的函数来实现图片验证码生成功能，有需要的朋友可以参考一下。
//must start or continue session and save CAPTCHA string in $_SESSION for
//it to be available to other requests
if(!isset($_SESSION)){
session_start();
header('Cache-control:private');
}
//create a 65*20 pixel image
$width=65;
$height=20;
$image=imagecreate(65,20);
//fill the image background color
$bg_color=imagecolorallocate($image,0x33,0x66,0xFF);
imagefilledrectangle($image,0,0,$width,$height,$bg_color);
//fetch random text
$text=random_text(5);
//determine x and y coordinates for centering text
$font=5;
$x=imagesx($image)/2-strlen($text)*imagefontwidth($font)/2;
$y=imagesy($image)/2-imagefontheight($font)/2;
//write text on image
$fg_color=imagecolorallocate($image,0xFF,0xFF,0xFF);
imagestring($image,$font,$x,$y,$text,$fg_color);
//save the CAPTCHA string for later comparison
$_SESSION['captcha']=$text;
//output the image
header('Content-type:image/png');
imagepng($image);
imagedestroy($image);
?>
