/**
 * 验证码类，用于产生各种验证码
 */
class captcha{
 
 
    /**
     * 生成验证码
     * @param int $length 码字符长度
     * @param int $width 宽度
     * @param int $height 长度
     * @param string $type 类型
     * @return 
     */
    public static function getCode($length=4,$width=50,$height=22,$type='png') 
    {
        //session_start();
        $randval = self::randString($length);
        //$_SESSION['captcha']= $randval;
         
        $width = ($length*9+10)>$width?$length*9+10:$width;
        if ( $type!='gif' && function_exists('imagecreatetruecolor')) {
            $im = @imagecreatetruecolor($width,$height);
        }else {
            $im = @imagecreate($width,$height);
        }
        $r = Array(225,255,255,223);
        $g = Array(225,236,237,255);
        $b = Array(225,236,166,125);
        $key = mt_rand(0,3);
 
        $backColor = imagecolorallocate($im, $r[$key],$g[$key],$b[$key]);    //背景色（随机）
        $borderColor = imagecolorallocate($im, 100, 100, 100);                    //边框色
        $pointColor = imagecolorallocate($im,mt_rand(0,255),mt_rand(0,255),mt_rand(0,255));            //点颜色
        @imagefilledrectangle($im, 0, 0, $width - 1, $height - 1, $backColor);
        @imagerectangle($im, 0, 0, $width-1, $height-1, $borderColor);
        $stringColor = imagecolorallocate($im,mt_rand(0,200),mt_rand(0,120),mt_rand(0,120));
        // 干扰
        for($i=0;$i<10;$i++){
            $fontcolor=imagecolorallocate($im,mt_rand(0,255),mt_rand(0,255),mt_rand(0,255));
            imagearc($im,mt_rand(-10,$width),mt_rand(-10,$height),mt_rand(30,300),mt_rand(20,200),55,44,$fontcolor);
        }
        for($i=0;$i<25;$i++){
            $fontcolor=imagecolorallocate($im,mt_rand(0,255),mt_rand(0,255),mt_rand(0,255));
            imagesetpixel($im,mt_rand(0,$width),mt_rand(0,$height),$pointColor);
        }
        @imagestring($im, 10, 20, 10, $randval, $stringColor);
 
           header("Content-type: Image/".$type);
        $ImageFun='Image'.$type;
        $ImageFun($im);
        imagedestroy($im); 
        return $randval;
    }
 
}

以上是完整的生成图片验证码的方法，下面是使用该方法生成验证码：

 include "../lib/captcha.class.php";
 $captcha = new captcha();
 $checkCode = $captcha->getCode(4,75,32);
 echo $checkCode;

最后在页面显示的效果就是这样，刷新一次重新生成一个：