Creating some equivalence class and border class in laravel for a password or name

I am new in testing and I have a new to create some classes in Laravel where to declare some properties. For example if i have a password input, i have to create some classes which are for equivalence and border testing. Like is should be aplhanumeric, more than 8 digits.

Can somebody help me understand better ? I mean if I create a class named AlphaNumeric, how am I going to put the property that this class needs to be alphanumeric ?

Thank you