Here is some example code for implementing a CAPTCHA, using the gd extension to create a server-rendered, randomized image.

```php
<?php
// Start the session to store the CAPTCHA answer
session_start();

// Define the image dimensions
$width = 200;
$height = 60;

// Create the image resource
$image = imagecreatetruecolor($width, $height);

// Define colors
$bg_color = imagecolorallocate($image, 255, 255, 255); // White background
$text_color = imagecolorallocate($image, 0, 0, 0);      // Black text
$noise_color = imagecolorallocate($image, 100, 100, 100); // Gray for noise

// Fill the background
imagefill($image, 0, 0, $bg_color);

// Generate a random math equation (e.g., 3 + 5)
$num1 = rand(1, 9);
$num2 = rand(1, 9);
$operator = rand(0, 1) ? '+' : '-'; // Randomly choose addition or subtraction
$equation = "$num1 $operator $num2 = ?";
$answer = $operator == '+' ? ($num1 + $num2) : ($num1 - $num2);

// Store the answer in the session
$_SESSION['captcha'] = $answer;

// Add noise to the image (random dots)
for ($i = 0; $i < 1000; $i++) {
    imagesetpixel($image, rand(0, $width), rand(0, $height), $noise_color);
}

// Set the path to the ZXX font
$font = __DIR__ . '/fonts/ZXX-Regular.ttf'; // Path to the ZXX font file

// Add the math equation using ZXX font
$font_size = 30;
$x = 20;
$y = 40;
imagettftext($image, $font_size, rand(-10, 10), $x, $y, $text_color, $font, $equation);

// Output the image as PNG
header('Content-Type: image/png');
imagepng($image);

// Free the image resource
imagedestroy($image);
?>
```

https://github.com/kawaiidesune/zxx