To install php55-mbstring, run the following command in macOS terminal (Applications->Utilities->Terminal)
sudo port install php55-mbstring
To see what files were installed by php55-mbstring, run:
port contents php55-mbstring
To later upgrade php55-mbstring, run:
sudo port selfupdate && sudo port upgrade php55-mbstring
Reporting an issue on MacPorts Trac
The MacPorts Project uses a system called Trac to file tickets to report bugs and enhancement requests.
Though anyone may search Trac for tickets, you must have a GitHub account in order to login to Trac to create tickets.