Unfortunately, the mcrypt extension is not supported in PHP 7.2 and later versions, including PHP 8.0. However, there are alternative solutions that you can use to encrypt and decrypt data in PHP.

One popular alternative is the openssl extension, which provides encryption and decryption functions using the OpenSSL library. Here’s an example of how you can use openssl_encrypt and openssl_decrypt functions to encrypt and decrypt data in PHP:

$key = 'mysecretkey';
$data = 'Hello, world!';

// Encrypt data
$ivlen = openssl_cipher_iv_length($cipher = "AES-128-CBC");
$iv = openssl_random_pseudo_bytes($ivlen);
$ciphertext = openssl_encrypt($data, $cipher, $key, $options=0, $iv);

// Decrypt data
$original_plaintext = openssl_decrypt($ciphertext, $cipher, $key, $options=0, $iv);

echo $original_plaintext; // Output: Hello, world!

Alternatively, you can use third-party encryption libraries like Sodium, which is a modern and easy-to-use library for encryption, decryption, signatures, and password hashing. Here’s an example of how you can use the sodium_crypto_secretbox and sodium_crypto_secretbox_open functions to encrypt and decrypt data using Sodium:

$key = random_bytes(SODIUM_CRYPTO_SECRETBOX_KEYBYTES);
$data = 'Hello, world!';

// Encrypt data
$nonce = random_bytes(SODIUM_CRYPTO_SECRETBOX_NONCEBYTES);
$ciphertext = sodium_crypto_secretbox($data, $nonce, $key);

// Decrypt data
$original_plaintext = sodium_crypto_secretbox_open($ciphertext, $nonce, $key);

echo $original_plaintext; // Output: Hello, world!

I hope this helps! Let me know if you have any other questions.