src/Core/CoreConstants.php
public static function getRefreshTokenFromArray(array $settings){
        if(array_key_exists('refreshTokenKey', $settings)){
            return $settings['refreshTokenKey'];
        }else if(array_key_exists('refreshToken', $settings)){
            return $settings['refreshToken'];
        } else if(array_key_exists('RefreshToken', $settings)){
            return $settings['RefreshToken'];
        } else{
            return null;
        }
    }
public static function getRefreshTokenFromArray(array $settings){
        if(array_key_exists('refreshTokenKey', $settings)){
            return $settings['refreshTokenKey'];
        }else if(array_key_exists('refreshToken', $settings)){
            return $settings['refreshToken'];
        } else if(array_key_exists('RefreshToken', $settings)){
            return $settings['RefreshToken'];
        } else{
            return null;
        }
    }
A helper method to get Refresh Token Key. People may have a typo on the key name
@param Array $settings     The array contains all the key values
@return String | null      The refresh token developer provided