<?php
    /**
     * Created by PhpStorm.
     * User: Administrator
     * Date: 2019/10/28
     * Time: 13:29
     */
     
     //image.func.php
    require_once "string.func.php";
    //通过GD库做验证码
    //创建画布
    $width = 80;
    $height = 28;
    $image = imagecreatetruecolor($width,$height);
    $white = imagecolorallocate($image,255,255,255);
    $black = imagecolorallocate($image,0,0,0);

    //用矩形填充画布
    imagefilledrectangle($image,1,1,$width-2,$height-2,$white);
   // $type = 1;
    //$length = 4;
    $chars = buildRandomString($type,$length);
    $sess_name = "verify";
    $fontfiles = array("dutch.ttf","dutchb.ttf","dutchbi.ttf","dutcheb.ttf","dutchi.ttf","tt0524m_.ttf","tt0586m_.ttf","tt1018m_.ttf");
    $_SESSION[$sess_name] = $chars;
    for($i=0;$i<$length;$i++){
        $size = mt_rand(14,18);
        $angle = mt_rand(-15,15);
        $x = 5+$i*$size;
        $y = mt_rand(20,26);
        $fontfile = "../fonts/".$fontfiles[mt_rand(0,$fontfiles)-1];
        $color = $imagecolorallocate($image,mt_rand(50,90),mt_rand(80,200),mt_rand(90,180));
        $text = substr($chars,$i,1);
        imagettftext($image,$size,$angle,$x,$y,$color,$fontfile,$text);
    }
    header("content-type:image/gif");
    imagegif($image);
    imagedestroy($image);

//string.func.php
<?php
    /**
     * Created by PhpStorm.
     * User: Administrator
     * Date: 2019/10/28
     * Time: 13:29
     */
    //生成随机字符串
    function buildRandomString($type = 1,$length = 4){
        if($type == 1){
            $chars = join("",range(0,9));
        }elseif($type == 2){
            $chars = join("",array_merge(range("a","z"),range("A","Z")));
        }elseif($type == 3){
            $chars = join("",array_merge(range("a","z"),range("A","Z"),range(0,9)));
        }
        if($length>strlen($chars)){
            exit("字符串长度不够");
        }
        $chars = str_shuffle($chars);
       return substr($chars,0,$length);
    }