PHP动态图像无法正常显示

Question

I just moved a site to a new server running PHP 7. It was originally on PHP 5.4, I believe. The following URL should display an image with a code in it, like a captcha:

http://appreviewhelper.com/static/captcha/GkK9yEiaw6

When that URL is hit, the following code is ran:

$code=rand(1000,9999);
$code = (string)$code;
$_SESSION["code"]=$code;

$im = imagecreate(50, 24);
$bg = imagecolorallocate($im, 238, 238, 238); //background color blue
$fg = imagecolorallocate($im, 85, 85, 85);//text color white
imagefill($im, 0, 0, $bg);
imagestring($im, 5, 5, 5,  $code, $fg);

header("Cache-Control: no-cache, must-revalidate");
header('Content-type: image/png');
header("Content-Disposition: inline; filename=captcha.png");
imagepng($im);

imagedestroy($im);

exit();

It was working fine on the old server. I've made sure the PHP GD library is installed and the module has been enabled, and I've verified that over and over... but, still I just get a small, blank image. Any ideas?

Thanks

Edit: Below is output from gd_info() from this site:

    array(12) {
  ["GD Version"]=>
  string(5) "2.1.1"
  ["FreeType Support"]=>
  bool(true)
  ["FreeType Linkage"]=>
  string(13) "with freetype"
  ["GIF Read Support"]=>
  bool(true)
  ["GIF Create Support"]=>
  bool(true)
  ["JPEG Support"]=>
  bool(true)
  ["PNG Support"]=>
  bool(true)
  ["WBMP Support"]=>
  bool(true)
  ["XPM Support"]=>
  bool(true)
  ["XBM Support"]=>
  bool(true)
  ["WebP Support"]=>
  bool(true)
  ["JIS-mapped Japanese Font Support"]=>
  bool(false)
}

Answer 1

I'm answering myself because, after hours and hours, I found the problem. The client shared the files with me using Dropbox. Apparently, Dropbox converted the files to Unicode. So, the file which had the image generation code was sending a BOM before the image was output, causing the image to look corrupt. Changing the encoding of the file to UTF-8 fixed the issue...

Thanks for the time of those who viewed and responded.