PHP7.1 and Laravel 5.3: Function mcrypt_get_iv_size() is deprecated

This error occurs because you probably have something other than AES-256-CBC as your cipher in your config/app.php file that depends on the mcrypt extension. Perhaps you are using MCRYPT_RIJNDAEL_256 or MCRYPT_RIJNDAEL_128?

The best thing you can do without a full-blown Laravel upgrade is install the legacy encrypter and use it to update all your encrypted data to use the AES-256-CBC cipher which has been the default cipher since Laravel 5.1, I believe. Once you do this, you should be able to use PHP 7.1 for your Laravel application.

Add this to the beginning of the config/app.php:

error_reporting(E_ALL ^ E_DEPRECATED);

Source: https://stackoverflow.com/a/42515505/225790