WordPress WooCommerce SagePay Form extension https://woocommerce.com/products/sage-pay-form/ requires the PHP mcrypt extension to operate.
If your site uses this extension and one day stops going through the full checkout process be aware of the following:
mcrypt has been deprecated as of PHP 7.1.0 and will be moved to PECL as of PHP 7.2.0 – this is because it relies upon libmcrypt which is dead and unmaintained since 2007 http://php.net/manual/en/intro.mcrypt.php.
Your host may have disabled the mcrypt extension.
I’ve heard that the WooCommerce SagePay Form extension is currently being updated to use openssl_encrypt so there may be an update soon to rectify this.