Detailed explanation of how PHP randomly generates image verification codes

session_start();
//Randomly generate a string of verification codes
function random($len) {
 $srcstr="ABCDEFGHIJKLMNPQRSTUVWXYZ123456789";
 mt_srand();
 $strs=" ";
 for($i=0;$i<$len;$i++) {
 $strs.=$srcstr[mt_rand(0,33)];
 }
 return ($strs);
}
$str =random(5); //Randomly generated string
$width=60; //Width of the verification code image
$height=25; //Height of the verification code image
//Date in the past
header(" Expires:Mon,26 Jul 1997 05:00:00 GMT");
//always modified
header("Last-Modified:".gmdate("D,d M Y H:i:s")."GMT");
//HTTP/1.1
header("Cache-Control:no-store,no-cache,must-revalidate");
header("Cache-Control:post-check=0,pre-check=0",false );
//HTTP/1.0
header("Pragma:no-cache");
header("Content-Type:image/png");

$im=imagecreate($width,$height);
$ back=imagecolorallocate($im,0xFF,0xFF,0xFF); //Background color
$pix=imagecolorallocate($im,187,190,247); //Blurry point color
$font=imagecolorallocate($im,41,163,238); //Font Color
//Draw 1000 blurred points
mt_srand();
for($i=0;$i<1000;$i++) {
imagesetpixel($im,mt_rand(0,$width),mt_rand(0 ,$height),$pix);
}
imagestring($im,5,7,5,$str,$font);//Draw a randomly generated string
imagerectangle($im,0,0,$width -1,$height-1,$font);//Draw a 1px border around the verification code image
imagepng($im);//Create a PNG format graphic
imagedestroy($im);//Handle the image Deconstruct and release in memory space
$_SESSION["auth_code"]=$str;
?>

Example 2, PHP random verification code php generates simple verification code

 $image_width=140;
 $image_height=50;
 srand(microtime()*10000);
 for($i=0;$i<4;$i++){
 $number.=dechex(rand(0,15));
 }
 $check=$number;
 $im=imagecreate($image_width,$image_height);
 imagecolorallocate($im,255,255,255);
 for($i =0;$i $font=rand(40,80);
 $x=rand(1,8)+$image_width*$i/4;
 $y= rand(1,$image_height/4);
 $color=imagecolorallocate($im,rand(0,100),rand(0,150),rand(0,200));
 imagestring($im,$font,$x,$y, $check[$i],$color);
 }
imagepng($im);
imagedestroy($im);
?>