为什么检测验证码时，浏览器只显示白色正方形边框

$width=200;
$height=50;
$image=imagecreatetruecolor($width,$height);
$white=imagecolorallocate($image,255,255,255);

imagefilledrectangle($image,0,0,$width,$height,$white);
$randColor=imagecolorallocate($image,mt_rand(0,255),mt_rand(0,255),mt_rand(0,255));

$size=mt_rand(20,28);
$angle=mt_rand(-15,15);
$x=50;
$y=30;
$fontFile='fonts/STSONG.TTF';
$text=mt_rand(1000,9999);
imagettftext($image,$size,$angle,$x,$y,$randColor,$fontFile,$text);

header('content-type:image/png');
imagepng($image);
imagedestroy($image);