php8.1网站后台不显示验证码,但是php5.6就可以显示

Question

在php5.6进入后台就可以显示代码，但在php8.1下就不显示

<input name="captcha" id="captcha" type="text" size="6" placeholder="验证码" class="input-captcha"/>&nbsp;<img src="{DT_PATH}api/captcha.png.php?action=image" title="验证码,看不清楚?请点击刷新&#10;字母不区分大小写" alt="" align="absmiddle" id="captchapng" onclick="reloadcaptcha();" style="cursor:pointer;"/><span id="ccaptcha"></span>
<script type="text/javascript">
function reloadcaptcha() {
    Dd('captchapng').src = '{DT_PATH}api/captcha.png.php?action=image&refresh='+Math.random();
    Dd('ccaptcha').innerHTML = '';
    Dd('captcha').value = '';
}
function checkcaptcha(s) {
    s = $.trim(s);
    var t = encodeURIComponent(s);
    if(t.indexOf('%E2%80%86') != -1) s = decodeURIComponent(t.replace(/%E2%80%86/g, ''));
    if(!is_captcha(s)) return;
    $.post(AJPath, 'action=captcha&captcha='+s,
        function(data) {
            if(data == '0') {
                Dd('ccaptcha').innerHTML = '&nbsp;&nbsp;<img src="{DT_STATIC}file/image/check-ok.png" align="absmiddle"/>';
                {if $DT_MOB == 'ios'}
                Dd('captcha').value = s;
                {/if}
            } else {
                Dd('captcha').focus;
                Dd('ccaptcha').innerHTML = '&nbsp;&nbsp;<img src="{DT_STATIC}file/image/check-ko.png" align="absmiddle"/>';
            }
        }
    );
}
function is_captcha(v) {
    if(v.match(/^[a-z0-9A-Z]{1,}$/)) {
        return v.match(/^[a-z0-9A-Z]{4,}$/);
    } else {
        return v.length > 1;
    }
}
$(function() {
    $('#captcha').bind('keyup blur', function() {
        checkcaptcha($('#captcha').val());
    });
});
</script>

<?php
/*
    [DESTOON B2B System] Copyright (c) 2008-2018 www.destoon.com
    This is NOT a freeware, use is subject to license.txt
*/
defined('IN_DESTOON') or exit('Access Denied');
class captcha {
    var $chars = 'abcdeghkmnpqstwxyz234789ABCEFGHJKLMNPRSTWXYZ';
    var $length = 4;
    var $soundtag;
    var $soundstr;
    var $cn;
    var $font;

    function question($id) {
        $r = DB::get_one("SELECT * FROM ".DT_PRE."question ORDER BY rand()");
        $_SESSION['answerstr'] = encrypt($r['answer'], DT_KEY.'ANS');
        exit('document.getElementById("'.$id.'").innerHTML = "'.$r['question'].'";');
    }

    function image() {
        if(strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE') !== false) {
            header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
            header('Pragma: public');
        } else {
            header('Pragma: no-cache');
        }
        header('Expires: '.gmdate('D, d M Y H:i:s').' GMT');
        header("Content-type: image/png");    
        $string = $this->mk_str();
        $_SESSION['captchastr'] = encrypt(strtoupper($string), DT_KEY.'CPC');
        $imageX = $this->length*26;
        $imageY = 32;
        $im = imagecreatetruecolor($imageX, $imageY);  
        imagefill($im, 0, 0, imagecolorallocate($im, 250, 250, 250));
        $color = imagecolorallocate($im, mt_rand(0, 100), mt_rand(0, 100), mt_rand(0, 100));
        if($this->cn) {
            $string = $string;
            $angle = mt_rand(-15, 15);
            $size = mt_rand(12, 22);
            $font = $this->font;
            $X = $size + mt_rand(5, 10);
            $Y = $size + mt_rand(5, 10);
            imagettftext($im, $size, $angle, $X, $Y, $color, $font, $string);
            $this->mk_sin($im, $color);
            imagepng($im);
            imagedestroy($im);
        } else {
            $fonts = glob(DT_ROOT.'/file/captcha/*.ttf');
            $num = count($fonts) - 1;
            $font = $fonts[mt_rand(0, $num)];
            $C0 = mt_rand(200, 255);
            $C1 = mt_rand(200, 255);
            $C2 = mt_rand(200, 255);
            $BG = imagecolorallocate($im, $C0, $C1, $C2);
            imagefill($im, 0, 0, $BG);
            $X = 0;
            for($i = 0; $i < $this->length; $i++) {
                $size = mt_rand(20, 25);
                $angle = mt_rand(-3, 3);
                if($i > 0) $X += $size - mt_rand(3, 5);
                $Y = $size + mt_rand(-2, 2);
                imagettftext($im, $size, $angle, $X, $Y, $color, $font, $string{$i});
            }
            $IM = imagecreatetruecolor($imageX, $imageY);
            imagefill($IM, 0, 0, $BG);                
            for($i = 0; $i < $imageX; $i++) {
                for($j = 0; $j < $imageY; $j++) {
                    $C = imagecolorat($im, $i, $j);
                    if(($i+20+sin($j/$imageY*2*M_PI)*6) <= $imageX && ($i+20+sin($j/$imageY*2*M_PI)*6) >=0 ) {
                        imagesetpixel($IM, $i+10+sin($j/$imageY*2*M_PI-M_PI*0.1)*0.8, $j, $C);
                    }
                }
            }
            $this->mk_sin($IM, $color);
            imagepng($IM);
            imagedestroy($IM);
            imagedestroy($im);
        }
        exit;
    }

    function mk_sin($im, $color) {
        $R = mt_rand(5, 20);
        $X = mt_rand(15, 25);
        $Y = mt_rand(5, 10);
        $L = mt_rand(50, 80);
        for($yy = $R; $yy <= $R + 1; $yy++) {
            for($px = -$L; $px <= $L; $px = $px + 0.1) {
                $x = $px/$X;
                if($x != 0) $y = sin($x);
                $py = $y*$Y;
                imagesetpixel($im, $px + $L, $py + $yy, $color);
            }
        }
    }

    function mk_str() {
        $str = '';
        if($this->cn) {
            $step = DT_CHARSET == 'UTF-8' ? 3 : 2;
            $text = substr(file_get(DT_ROOT.'/file/config/cncaptcha.inc.php'), 13);
            $max = strlen($text) - 1 - $step;
            while(1) {
                $i = mt_rand(0, $max);
                if($i%$step == 0) {
                    $str .= substr($text, $i, $step);
                    break;
                }
            }
            while(1) {
                $i = mt_rand(0, $max);
                if($i%$step == 0) {
                    $str .= substr($text, $i, $step);
                    break;
                }
            }
        } else {
            $max = strlen($this->chars) - 1;
            while(1) {
                if(strlen($str) == $this->length) break;
                $r = mt_rand(0, $max);
                if(strpos(strtolower($str), strtolower($this->chars{$r})) === false) $str .= $this->chars{$r};
            }
        }
        return $str;
    }
}
?>