PHP simple high performance captcha (uses memcache)

captcha_readme.txt

Include AHGBold.tff
Run memcached at default port (11211)

Example captcha: https://i.ibb.co/sPQjW0K/index.png

show_captcha.php

<?php
$captchaSecret = "123";

# Generate captcha
$image = imagecreatetruecolor(200, 50);       
$background_color = imagecolorallocate($image, 255, 255, 255);  
imagefilledrectangle($image,0,0,200,50,$background_color); 

$number_of_lines=rand(3,7);

for($i=0;$i<$number_of_lines;$i++)
{
    $line_color = imagecolorallocate($image, rand(0,255), rand(0,255), rand(0,255));
    imageline($image,0,rand()%50,250,rand()%50,$line_color);
}

for($i=0;$i<500;$i++)
{
    $pixel = imagecolorallocate($image, rand(0,255),rand(0,255),rand(0,255));
    imagesetpixel($image,rand()%200,rand()%50,$pixel);
}  

$allowed_letters = 'ABCDEFGHJKLMNPQRSTUVWXYZabcdefghjkmnopqrstuvwxyz123456789';
$length = strlen($allowed_letters);
$letter = $allowed_letters[rand(0, $length-1)];
$word='';
$text_color = imagecolorallocate($image, 0,0,0);
$cap_length=6;// No. of character in image
$lastx = -10;
for ($i = 0; $i< $cap_length;$i++)
{
    $color = imagecolorallocate($image, rand(0, 255), rand(0, 180), rand(0,180));
    $letter = $allowed_letters[rand(0, $length-1)];
    $lastx += rand(20,33);
    imagettftext($image, rand(17,20), rand(0, 35), $lastx, rand(25,40) , $color, "AHGBold.ttf" , $letter );
    $word.=$letter;
}

# put captcha to a variable as base64
ob_flush();
ob_start();
imagepng($image);
$img = ob_get_clean();
ob_end_clean();
imagedestroy($image);

$b64encode = base64_encode($img);

$validUntil = time()+60;
$token = sha256(strtolower($word).sha256($captchaSecret.$validUntil));

# show captcha
?>

<img src="data:image/png;base64,<?=$b64encode?>">
<form action="verify_captcha.php" method="post">
    <input type="text" name="captchaValue" minlength="6" maxlength="6" class="form-control" placeholder="">
    <input type="hidden" name="validUntil" value="<?=$validUntil?>">
    <input type="hidden" name="token" value="<?=$token?>">
    <button type="submit" class="btn btn-primary btn-block">Submit</button>
</form>

verify_captcha.php

<?php
$captchaSecret = "123";

$validUntil = $_POST['validUntil'];
$captchaValue = $_POST['captchaValue'];
$originaltoken = $_POST['token'];

$delta = $validUntil - time();
if($delta > 60)
	throw new CustomException('Captcha is invalid');
else if($delta < 0)
	throw new CustomException('Captcha is expired');

if(strlen($originaltoken) != 64)
	throw new CustomException('Captcha is invalid');
if(strlen($captchaValue) != 6)
	throw new CustomException('Wrong captcha');

$memcached = new Memcached;
$memcached->addServer("127.0.0.1", 11211);
# if token is used or memcached server is down
if($memcached->get($originaltoken.":used") === true || $memcached->getResultCode() !== 16)
	throw new CustomException('Captcha is expired');

$memcached->set($originaltoken.":used", true, 70);

$solvedtoken = sha256(strtolower($captchaValue).sha256($captchaSecret.$validUntil));
if($solvedtoken == $originaltoken){
	session_start();
	$_SESSION['captchatimeout'] = time()+1800;
  # create a session to keep user authenticated
  die("Captcha verified!");
}
else{
	throw new CustomException('Wrong captcha');
}