大概代码如下

$im = imagecreate(60,22);
$black = ImageColorAllocate($im, 0,0,0);
$white = ImageColorAllocate($im, 255,255,255);
$gray = ImageColorAllocate($im, 200,200,200);
$red=imageColorAllocate($im,255,0,0);
$randcolor=ImageColorAllocate($im,rand(0,255),rand(0,255),rand(0,255));
imagefill($im,0,0,$gray);
imagestring($im, 5, 10, 5, $checknum, $black);

就是极普通的数字验证码