PHP validation class for username, email, and password
Well, your class is good and does what it is supposed to do, except in validalidatecheckbox() (you are using variables that doesn't exist).

Also I would do this changes:


Remove this code (it's unused):

private $password,
    $repeatPassword,
    $username,
    $email;
Change this to constants, so you can use it elsewhere:

private static $minPassword = 7,
    $confirmPassword,
    $minUsername = 3,
    $maxUsername = 14,
    $validEmail;


So your class should look like this:

class Validation
{
    const MIN_PASSWORD_LENGTH = 7;
    const MIN_USERNAME_LENGTH = 3;
    const MAX_USERNAME_LENGTH = 14;

    // rest of your code


Also, I strongly recommend you to check PSR-1 and PSR-2 to make your code look standard-like.