How to implement random number and letter verification code in php

<?php
Class Captcha{
        private $_fontfile=&#39;&#39;;
        private $_size=36;
        private $_width=200;
        private $_height=100;
        private $_length=4;
        private $_image=null;
        private $_snow=0;
        private $_pixel=0;
        private $_line=0;
    public function __construct($config=array()){
        if(is_array($config)&&count($config)>0){
            if(isset($config[&#39;fontfile&#39;])&&is_file($config[&#39;fontfile&#39;])&&is_readable($config[&#39;fontfile&#39;])){
                $this->_fontfile=$config[&#39;fontfile&#39;];
            }else{
                return false;
            }
            if(isset($config[&#39;size&#39;])&&$config[&#39;size&#39;]>0){
                $this->_size=(int)$config[&#39;size&#39;];
            }
            if(isset($config[&#39;width&#39;])&&$config[&#39;width&#39;]>0){
                $this->_width=(int)$config[&#39;width&#39;];
            }
            if(isset($config[&#39;height&#39;])&&$config[&#39;height&#39;]>0){
                $this->_height=(int)$config[&#39;height&#39;];
            }
            if(isset($config[&#39;length&#39;])&&$config[&#39;length&#39;]>0){
                $this->_length=(int)$config[&#39;length&#39;];
            }
            if(isset($config[&#39;snow&#39;])&&$config[&#39;snow&#39;]>0){
                $this->_snow=(int)$config[&#39;snow&#39;];
            }
            if(isset($config[&#39;pixel&#39;])&&$config[&#39;pixel&#39;]>0){
                $this->_pixel=(int)$config[&#39;pixel&#39;];
            }
            if(isset($config[&#39;line&#39;])&&$config[&#39;line&#39;]>0){
                $this->_line=(int)$config[&#39;line&#39;];
            }
            $this->_image=imagecreatetruecolor($this->_width,$this->_height);
            return $this->_image;

         }
         else{
            return false;
        }
    }
    public function getCaptcha(){
        $white=imagecolorallocate($this->_image,255,255,255);
        imagefilledrectangle($this->_image,0,0,$this->_width,$this->_height,$white);
        $str=$this->_generateStr($this->_length);
        if(false===$str){
            return false;
        }
        $fontfile=$this->_fontfile;
        for($i=0;$i<$this->_length;$i++){
            $size=$this->_size;
            $angle=mt_rand(-30,30);
            $x=ceil($this->_width/$this->_length)*$i+mt_rand(5,10);
            $y=ceil($this->_height/1.5);
            $color=$this->_getRandColor();
            //针对中文字符截取
            //$text=mb_substr($str,$i,1,&#39;utf-8&#39;);
            $text=$str{$i};
            imagettftext($this->_image, $size, $angle, $x, $y, $color, $fontfile, $text);
        }
        if($this->_snow){
            $this->_getSnow();

        }else{
            if($this->_pixel){
                $this->_getPixel();

            }
            if($this->_line){
                $this->_getLine();
            }
        }
        header(&#39;content-type:image/png&#39;);
        imagepng($this->_image);
        imagedestroy($this->_image);
        return strtolower($str);
    }
    private function _getSnow(){
        for($i=1;$i<=$this->_snow;$i++){
            imagestring($this->_image,mt_rand(1,5),mt_rand(0,$this->_width),mt_rand(0,$this->_height),&#39;*&#39;,$this->_getRandColor());
        }
    }
    private function _getPixel(){
        for($i=1;$i<=$this->_pixel;$i++){
            imagesetpixel($this->_image,mt_rand(0,$this->_width),mt_rand(0,$this->_height),$this->_getRandColor());
        }
    }
    private function _getLine(){
        for($i=1;$i<=$this->_line;$i++){
            imageline($this->_image,mt_rand(0,$this->_width),mt_rand(0,$this->_height),mt_rand(0,$this->_width),mt_rand(0,$this->_height),$this->_getRandColor());

        }
    }
    private function _generateStr($length=4){
        if($length<1 || $length>30){
            return false;
        }
        $chars=array(
            &#39;a&#39;,&#39;b&#39;,&#39;c&#39;,&#39;d&#39;,&#39;e&#39;,&#39;f&#39;,&#39;g&#39;,&#39;h&#39;,&#39;k&#39;,&#39;m&#39;,&#39;n&#39;,&#39;p&#39;,&#39;x&#39;,&#39;y&#39;,&#39;z&#39;,
            &#39;A&#39;,&#39;B&#39;,&#39;C&#39;,&#39;D&#39;,&#39;E&#39;,&#39;F&#39;,&#39;G&#39;,&#39;H&#39;,&#39;K&#39;,&#39;M&#39;,&#39;N&#39;,&#39;P&#39;,&#39;X&#39;,&#39;Y&#39;,&#39;Z&#39;,
            1,2,3,4,5,6,7,8,9
            );
        $str=join(&#39;&#39;,array_rand(array_flip($chars),$length));
        return $str;
    }
    private function _getRandColor(){
        return imagecolorallocate($this->_image,mt_rand(0,255),mt_rand(0,255),mt_rand(0,255));
    }
}
?>

<?php
require_once &#39;classgd.class.php&#39;;
$config=array(
&#39;fontfile&#39;=>&#39;fonts/simfang.ttf&#39;,   //引入字体文件
//&#39;snow&#39;=>50,
&#39;pixel&#39;=>100,
&#39;line&#39;=>10
    );
$captcha=new Captcha($config);
$captcha->getCaptcha();
?>