以官网文档为例,这段代码在php7.2以前都能正常显示。

php7.2就不能显示了。因为字体文件路径的原因，不能被正确的识别

要么按照官方文档的方法

将当前路径放到环境变量中，要么使用 __DIR__ ，__FILE__，realpath函数等得到字体的物理路径。如

/**
     * 输出验证码并把验证码的值保存的session中
     * 验证码保存到session的格式为： array('verify_code' => '验证码值', 'verify_time' => '验证码创建时间');
     * @access public     
     * @param string $id 要生成验证码的标识   
     * @return void
     */
    public function entry($id = '') {
        // 图片宽(px)
        $this->imageW || $this->imageW = $this->length*$this->fontSize*1.5 + $this->length*$this->fontSize/2; 
        // 图片高(px)
        $this->imageH || $this->imageH = $this->fontSize * 2.5;
        // 建立一幅 $this->imageW x $this->imageH 的图像
        $this->_image = imagecreate($this->imageW, $this->imageH); 
        // 设置背景      
        imagecolorallocate($this->_image, $this->bg[0], $this->bg[1], $this->bg[2]); 

        // 验证码字体随机颜色
        $this->_color = imagecolorallocate($this->_image, mt_rand(1,150), mt_rand(1,150), mt_rand(1,150));
        // 验证码使用随机字体
        $ttfPath = $this->_path . ($this->useZh ? 'zhttfs' : 'ttfs') . '/';

        if(empty($this->fontttf)){
            $dir = dir($ttfPath);
            $ttfs = array();		
            while (false !== ($file = $dir->read())) {
                if($file[0] != '.' && substr($file, -4) == '.ttf') {
                    $ttfs[] = $file;
                }
            }
            $dir->close();
            $this->fontttf = $ttfs[array_rand($ttfs)];
        } 
        $this->fontttf = realpath($ttfPath . $this->fontttf);//获取字体的物理路径

        if($this->useImgBg) {
            $this->_background();
        }
        
        if ($this->useNoise) {
            // 绘杂点
            $this->_writeNoise();
        } 
        if ($this->useCurve) {
            // 绘干扰线
            $this->_writeCurve();
        }
        
        // 绘验证码
        $code = array(); // 验证码
        $codeNX = 0; // 验证码第N个字符的左边距
        if($this->useZh){ // 中文验证码
            for ($i = 0; $i<$this->length; $i++) {
                $code[$i] = iconv_substr($this->zhSet,floor(mt_rand(0,mb_strlen($this->zhSet,'utf-8')-1)),1,'utf-8');
                imagettftext($this->_image, $this->fontSize, mt_rand(-40, 40), $this->fontSize*($i+1)*1.5, $this->fontSize + mt_rand(10, 20), $this->_color, $this->fontttf, $code[$i]);
            }
        }else{
            for ($i = 0; $i<$this->length; $i++) {
                $code[$i] = $this->codeSet[mt_rand(0, strlen($this->codeSet)-1)];
                $codeNX  += mt_rand($this->fontSize*1.2, $this->fontSize*1.6);
                imagettftext($this->_image, $this->fontSize, mt_rand(-40, 40), $codeNX, $this->fontSize*1.6, $this->_color, $this->fontttf, $code[$i]);
            }
        }
       
        // 保存验证码
        $key        =   $this->authcode($this->seKey);
        $code       =   $this->authcode(strtoupper(implode('', $code)));
        $secode     =   array();
        $secode['verify_code'] = $code; // 把校验码保存到session
        $secode['verify_time'] = NOW_TIME;  // 验证码创建时间
        session($key.$id, $secode);
                        
        header('Cache-Control: private, max-age=0, no-store, no-cache, must-revalidate');
        header('Cache-Control: post-check=0, pre-check=0', false);		
        header('Pragma: no-cache');
        header("content-type: image/png");

        // 输出图像
        imagepng($this->_image);
        imagedestroy($this->_image);
    }