# In create_user.php
* Create two methods, one to validate the input and one to actually persist into the DB
* Write both methods so that they accept a parameter instead of pulling from $_POST

* Have the validate method return an associative array of field => error. So like:

function validate($params){
// stuff here

return ["password" => "too short", "username" => "in use"];
}

# In index.php
* Call validate, and if there are errors stick them above the field with some Bootstrap errors ;)
* Put the original values of the form in if there's an error