Creating a CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) in PHP 7.4 involves generating an image with distorted text and then validating the user's input against that text. Below is a basic example of how to create a simple CAPTCHA using PHP 7.4:

1. Create a PHP file, e.g., captcha.php, to generate the CAPTCHA image and handle user input validation.

```php

session_start();

// Function to generate random string for CAPTCHA

function generateRandomString($length = 6) {

$characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';

$randomString = '';

for ($i = 0; $i < $length; $i++) {

$randomString .= $characters[rand(0, strlen($characters) - 1)];

}

return $randomString;

}

// Generate a random CAPTCHA string and store it in a session

$randomString = generateRandomString();

$_SESSION['captcha'] = $randomString;

// Set the image size for CAPTCHA

$imageWidth = 120;

$imageHeight = 40;

// Create the image resource

$captchaImage = imagecreatetruecolor($imageWidth, $imageHeight);

// Generate background color for the CAPTCHA image

$bgColor = imagecolorallocate($captchaImage, 255, 255, 255);

imagefill($captchaImage, 0, 0, $bgColor);

// Generate text color for the CAPTCHA image

$textColor = imagecolorallocate($captchaImage, 0, 0, 0);

// Add the random string to the CAPTCHA image

imagettftext($captchaImage, 18, rand(-10, 10), 25, 30, $textColor, 'path/to/your/font.ttf', $randomString);

// Add some random lines to make the CAPTCHA more difficult to read for bots

for ($i = 0; $i < 3; $i++) {

$lineColor = imagecolorallocate($captchaImage, rand(0, 255), rand(0, 255), rand(0, 255));

imageline($captchaImage, rand(0, $imageWidth), 0, rand(0, $imageWidth), $imageHeight, $lineColor);

}

// Output the CAPTCHA image

header('Content-type: image/png');

imagepng($captchaImage);

imagedestroy($captchaImage);

?>

```

2. Create an HTML form that includes the CAPTCHA image and allows the user to enter their response.

```html

CAPTCHA Example

```

3. Create another PHP file, e.g., validate_captcha.php, to validate the user's input against the CAPTCHA.

```php

session_start();

if (isset($_POST['captcha'])) {

$userInput = $_POST['captcha'];

// Retrieve the stored CAPTCHA value from the session

$captchaValue = isset($_SESSION['captcha']) ? $_SESSION['captcha'] : '';

// Validate the user's input against the stored CAPTCHA value

if ($userInput === $captchaValue) {

// Valid CAPTCHA, do your desired action (e.g., allow user to proceed)

echo "CAPTCHA validation successful!";

} else {

// Invalid CAPTCHA, handle the error (e.g., show an error message)

echo "CAPTCHA validation failed!";

}

// Clear the CAPTCHA value from the session after validation (optional but recommended)

unset($_SESSION['captcha']);

}

?>

```

In this example, we use the GD library functions to create an image with a random CAPTCHA string. We also add some random lines to make the CAPTCHA more difficult to read for bots. When the user submits the form, the input is validated against the stored CAPTCHA value from the session. If the input matches the CAPTCHA value, the CAPTCHA is considered valid.

Please note that this is a simple example for educational purposes. To create a more secure CAPTCHA, you may need to consider additional techniques like image obfuscation, time limitations, and rate limiting to prevent automated attacks effectively.