PHP object-oriented simple verification code class

<?php
class authCode
{
    private static $instance = null;     #实例对象
    private $width = 120;                #图片宽度
    private $height = 40;                #图片高度
    private $font = &#39;font/elephant.ttf&#39;; #字体文件路径
    private $fontSize = 14;              #字体大小
    private $strLen = 6;                 #字符个数
    private $auth_code_str = null;       #验证码结果
    private $imgResult = null;           #图片资源
    
    #入口文件 静态方法调用 实例化对象 可用 对象方法调用
    public static function img()
    {
        if (!(self::$instance instanceof self)) {
            self::$instance = new self();
        }
        return self::$instance;
    }
    
     #随机颜色
    private function randomColor($img = null, $min = 0, $max = 255)
    {
        $rgb = [];
        for ($i = 1; $i <= 3; $i++) {
            $rgb[] = str_pad(rand($min, $max), 3, 0, STR_PAD_LEFT);
        }
        return imagecolorallocate($img, $rgb[0], $rgb[1], $rgb[2]);
    }
    
    #随机字符串
    private function randomStr($num = 4)
    {
        if ($num > 0) {
            $string = array_merge(range(&#39;a&#39;, &#39;z&#39;), range(0, 9), range(&#39;A&#39;, &#39;Z&#39;), range(0, 9));
            for ($i = 1; $i <= $num; $i++) {
                shuffle($string);
                $this->auth_code_str .= array_pop($string);
            }
        }
        return $this;
    }
    
     #创建验证码
    public function createAuthCode(&$codeStr = false)
    {
        if (!$this->auth_code_str) {
            $this->randomStr($this->strLen);
        }
        if ($codeStr !== false && empty($codeStr)) {
            $codeStr = $this->auth_code_str;
        } else if (!empty($codeStr) && $codeStr !== false) {
            $this->auth_code_str = $codeStr;
        }
        $this->imgResult = imagecreatetruecolor($this->width, $this->height);
        $background = $this->randomColor($this->imgResult, 200);
        imagefilledrectangle($this->imgResult, 0, 0, $this->width, $this->height, $background);
        $y = ($this->height - $this->fontSize);
        $string = str_split($this->auth_code_str, 1);
        for ($i = 0; $i < count($string); $i++) {
            $frontColor = $this->randomColor($this->imgResult, 0, 200);
            imagefttext($this->imgResult, $this->fontSize, rand(0, 10), ($this->fontSize + 2) * $i + 10, $y, $frontColor, $this->font, $string[$i]);
        }
        return $this;
    }
    
      #生成线
    public function line($line = 3)
    {
        $line = $line ?: 3;
        for ($i = 1; $i <= $line; $i++) {
            $lineColor = $this->randomColor($this->imgResult, 0, 200);
            imageline($this->imgResult, rand(0, $this->width / 5), rand(5, $this->height - 5), rand($this->width / 1.3, $this->width), rand(5, $this->height - 5), $lineColor);
        }
        return $this;
    }
    
    #噪点
    public function pixel($num = 50){
        $num = $num ?: 3;
        for ($i = 1; $i <= $num; $i++) {
            $lineColor = $this->randomColor($this->imgResult, 0, 100);
            imagesetpixel($this->imgResult, rand(0, $this->width), rand(0, $this->height), $lineColor);
        }
        return $this;
    }
    
     #设置大小
    public function size($width = null, $height = null)
    {
        $this->width = $width ?: 120;
        $this->height = $height ?: 40;
        return $this;
    }
    
    #设置字体大小
    public function fontSize($fontsize = 14)
    {
        $this->fontSize = $fontsize ?: 14;
        return $this;
    }
    
    #设置字体
    public function font($file = null)
    {
        if (is_null($file) === true) {
            $this->font = &#39;font/elephant.ttf&#39;;
        } else {
            $this->font = $file;
        }
        return $this;
    }
    
    #设置长度
    public function strlen($num = null)
    {
        $this->strLen = $num ?: 6;
        return $this;
    }
    
    public function display()
    {
        ob_end_flush();
        header("content-type:image/jpeg");
        imagejpeg($this->imgResult, null, 100);
        imagedestroy($this->imgResult);
        exit;
    }
}

#简单调用方法
authCode::img()->createAuthCode()->display();
/*
#指定字符串调用
$string = &#39;abc123&#39;;
authCode::img()->createAuthCode($string)->display();
 
#设置图片大小、字数、字体大小
authCode::img()->strlen(8)->size(300,100)->fontSize(30)->createAuthCode()->display();
 
#添加噪点
authCode::img()->createAuthCode()->line()->pixel()->display();
*/
    ?>