In a D7 site using PHP 5.6, adding a key to the UI and selecting "Lockr" as the Key Provider results in the following error:
Fatal error: Call to undefined function Lockr\KeyWrapper\random_bytes() in /code/sites/all/modules/contrib/lockr/vendor/lockr/lockr/src/KeyWrapper/LockrAes256CbcSha256RawKeyWrapper.php on line 26
The random_bytes() function was introduced with PHP 7 - to fix this issue, replace it with drupal_random_bytes(), which works with PHP 5.6.