Homestead vagrant up Parse error

I've been using Laravel 5.2 and recently started switching to Laravel 5.5. After configuring the etc/hosts and Homestead.yaml file, I encountered the following error:

Symfony \ Component \ Debug \ Exception \ FatalThrowableError (E_PARSE)
Parse error: syntax error, unexpected '?', expecting variable (T_VARIABLE)

As many have mentioned about PHP version problem, I thought once I have the homestead running, it should take care of it? As the doc said, in includesPHP 7.2 PHP 7.1 PHP 7.0 PHP 5.6 (doc). Moreover I have PHP 7.1.16 installed, which should meet the Laravel 5.5's requirement(PHP>=7.0.0)

I also updated virtual box to 5.2 and Vagrant to 2.1.1. Also, when typing vagrant box list, I see this: laravel/homestead (virtualbox, 0.5.0)

Please help, thanks!!

via Chebli Mohamed