Encrypt/decrypt some options stored in the wp_options table.

encrypt_some_options.php

<?php
/**
 * Encrypt/decrypt some options stored in the wp_options table.
 * Note this will only work for options that are stored as strings.
 *
 * title: Encrypt/Decrypt Some Options
 * layout: snippet
 * collection: 
 * category: 
 * link: 
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

// Array of options to encrypt.
global $my_options_to_encrypt;
$my_options_to_encrypt = array(
    'pmpro_stripe_secretkey',
    'stripe_publishablekey',
);

// Set up filters to encrypt/decrypt options.
foreach( $my_options_to_encrypt as $option ) {
    add_filter( "pre_update_option_{$option}", 'my_maybe_encrypt', 10, 3 );
    add_filter( "option_{$option}", 'my_maybe_decrypt', 10 );
}

/**
 * Decrypt options before they are returned.
 */
function my_maybe_decrypt( $value ) {
    // Bail if already decrypted.
    if ( substr( $value, 0, 5 ) !== '$HASH' ) {
        return $value;
    }

    // Remove the prefix and decrypt.
    return my_decrypt( substr( $value, 5 ) );
}

/**
 * Encrypt options before they are saved.
 */
function my_maybe_encrypt( $new_value, $old_value, $option ) {
    // Bail if already encrypted.
    if ( substr( $new_value, 0, 5 ) === '$HASH' )
        return $new_value;

    return '$HASH' . my_encrypt( $new_value );
}

/**
 * A simple encryption function using OpenSSL.
 */
function my_encrypt($data, $key = null) {
    $key = $key ?: wp_salt();
    $iv = openssl_random_pseudo_bytes(16);
    $encrypted = openssl_encrypt($data, 'aes-256-cbc', $key, 0, $iv);
    return base64_encode($iv . $encrypted);
}

/**
 * A simple decryption function using OpenSSL.
 */
function my_decrypt($data, $key = null) {
    $key = $key ?: wp_salt();
    $data = base64_decode($data);
    $iv = substr($data, 0, 16);
    $encrypted = substr($data, 16);
    return openssl_decrypt($encrypted, 'aes-256-cbc', $key, 0, $iv);
}