Simple PHP verification image generation function_PHP tutorial

function yzm($name,$width,$height){
Header("Content-type: image/PNG");
srand((double)microtime()*1000000);//Sow a seed to generate random numbers to facilitate the use of random number generation below
session_start();//Save random numbers into the session
$_SESSION[$name]="";
$im = imagecreate($width,$height); //Set the image background size
$black = ImageColorAllocate($im, 0,0,0); //Set three colors
$white = ImageColorAllocate($im, 255,255,255);
$gray = ImageColorAllocate($im, 200,200,200 );
imagefill($im,0,0,$gray); //Use area filling method, set (0,0)
while(($authnum=rand()%100000)<10000 );
$_SESSION[$name]=$authnum;
imagestring($im, 5, 10, 3, $authnum, $black);
for($i=0;$i<200 ;$i++)//Add interference pixels
{
$randcolor = ImageColorallocate($im,rand(0,255),rand(0,255),rand(0,255));
imagesetpixel($im, rand ()%70 , rand()%30 , $randcolor);
}
ImagePNG($im);
ImageDestroy($im);
}?>