phpcms验证码不显示

function get_captcha() {
    $width = 160;
    $height = 40;
    $font_size = 20;
    $code_num = 4;
    $code = '';
    for ($i = 0; $i < $code_num; $i++) {
        $code .= chr(mt_rand(48, 57));
    }
    $_SESSION['captcha'] = $code;
    $img = imagecreatetruecolor($width, $height);
    $bg_color = imagecolorallocate($img, 255, 255, 255);
    imagefill($img, 0, 0, $bg_color);
    for ($i = 0; $i < $code_num; $i++) {
        $font_color = imagecolorallocate($img, mt_rand(0, 120), mt_rand(0, 120), mt_rand(0, 120));
        $x = ($width / $code_num) * $i + 5;
        $y = mt_rand(0, $height / 4);
        imagechar($img, $font_size, $x, $y, $code[$i], $font_color);
    }
    header('Content-type:image/png');
    imagepng($img);
    imagedestroy($img);
}

<img src="path/to/captcha.php?timestamp=<?php echo time(); ?>" alt="验证码" onclick="this.src='path/to/captcha.php?timestamp=<?php echo time(); ?>&refresh=1'" />

if ($_POST['captcha'] != $_SESSION['captcha']) {
    echo "验证码错误";
    exit;
}