What steps can be taken to transition from using MCrypt to OpenSSL or other modern PHP-based libraries for encryption, especially when updating both a website and a mobile app for compatibility?

MCrypt is deprecated in newer versions of PHP, so transitioning to OpenSSL or other modern PHP-based encryption libraries is necessary for security and compatibility reasons. To make this transition, you can update your encryption and decryption functions to use OpenSSL functions like `openssl_encrypt` and `openssl_decrypt`. This will ensure that your website and mobile app can communicate securely using modern encryption standards.

// Update encryption function using OpenSSL
function encryptData($data, $key) {
    $cipher = "AES-256-CBC";
    $ivlen = openssl_cipher_iv_length($cipher);
    $iv = openssl_random_pseudo_bytes($ivlen);
    $encrypted = openssl_encrypt($data, $cipher, $key, 0, $iv);
    return base64_encode($iv . $encrypted);
}

// Update decryption function using OpenSSL
function decryptData($data, $key) {
    $cipher = "AES-256-CBC";
    $ivlen = openssl_cipher_iv_length($cipher);
    $data = base64_decode($data);
    $iv = substr($data, 0, $ivlen);
    $data = substr($data, $ivlen);
    return openssl_decrypt($data, $cipher, $key, 0, $iv);
}