Thats a good php package / class !! loool
This is great !! lol
But ... how you check if password is correct ??
A good ideia is store encripted passwords in the database, using crypt or md5 functions ... !! lool
I see that you create a configuration file, this is a good practice, i'm using in any app one configuration script, or most if it has to many confs ... !! lol
And i recomend you to use constants, its better, it will be called in any point (code, in the functions, classes), otherwise you has 2 hypotheses:
1) declare glogal variables in the functions
2) pass variable value to function
Constants is better simple and easy ... !!