gistfile1.php

    public function rules()
    {
        return [
            ['username', 'required'],
            ['username', 'unique', 'message' => 'This username has already been taken.'],
            ['username', 'string', 'min' => 2, 'max' => 255],

            ['email', 'required'],
            ['email', 'email'],
            ['email', 'unique', 'message' => 'This email address has already been taken.'],

            ['password', 'string', 'min' => 6],

            ['fname', 'required'],
        ];
    }