Install these 3 PHP extension(s):

mbstring
curl
mysqli or mysql

You can usually install a PHP extension using apt-get or yum. Common package names are php-extname or php5-extname. Try commands like these:

$ sudo apt-get install php5-extname  # Debian / Ubuntu
$ sudo yum install php-extname       # Red Hat / Derivatives

If those commands don't work, try Google. The process of installing PHP extensions is not specific to this software, and any instructions you can find for installing them on your system should work. On Mac OS X, you might want to try Homebrew.

After installing new PHP extensions, restart everything for the changes to take effect. For help with restarting everything, see Restarting Phabricator in the documentation.