This is a shell script, it will not work from PHP until and unless you have shell access from your domain. If you have shell access it will work out.
If you don't have shell access get phpMyAdmin software upload the same and use export option select phpbb_ tables and download them to your local computer or to directory.
Download :
phpMyAdmin | MySQL Database Administration Tool | www.phpmyadmin.net
See how to backup tables with this software :
WordPress Backup with phpMyAdmin
Another option is use phpbb's own backup and restore table option.
Keeping in mind that if your tables are quite big, php may not work on shared hosting account due to restrictions.
Hope this helps!