Doesn't Sierra come preinstalled with Apache2 and PHP 5.6?
start apache2 if it isn't yet:
apachectl start
uncomment this line from httpd.conf (back it up) to enable php5:
LoadModule php5_module libexec/apache2/libphp5.so
then I guess:
apachectl restart
To get MySQL:
http://dev.mysql.com/downloads/mysql/
(don't forget to (optionally though) run mysql_secure_installation afterwards)
To let PHP and MySQL talk to eachother, I use this:
mkdir mysql under /var and go inside it with cd /var/mysql
ln -s /tmp/mysql.sock mysql.sock
I guess for XenForo you can then run the requirements.php file and test mysql to see if you can use it.
You probably need to edit httpd.conf and enable things like rewritemod.
LoadModule rewrite_module libexec/apache2/mod_rewrite.so
Anyway, once it's all working, before installing xenforo, setup a virtual host for your test site first.
Maybe add easier management things like phpmyadmin