$gray = imagecolorallocate($img,255,255);
$black = imagecolorallocate($img,rand(0,200),200));
$red = imagecolorallocate($img,0);
$white = imagecolorallocate($img,255);
$green = imagecolorallocate($img,0);
$blue = imagecolorallocate($img,255);
imagefilledrectangle($img,100,30,$black);

for($i = 0;$i < 80;$i++){
imagesetpixel($img,$w),$h),$gray);
}

$num1 = rand(1,99);
$num2 = rand(1,99);

imagestring($img,5,rand(1,10),$num1,$red);
imagestring($img,getRand(),$white);
imagestring($img,45,$num2,$green);
imagestring($img,65,"=",$blue);
imagestring($img,80,"?",$red);

header("content-type:image/png");
imagepng($img);
imagedestroy($img);
}
function getRand(){
$code = rand(0,1);
switch ($code) {
case 0:
return "+";
break;
case 1:
return "-";
break;
default:

code...

break;
}
}

?>