when installing phpmyadmin under hhvm and nginx especially, you might get an error like so:
“Error during session start; please check your PHP and/or webserver log file and configure your PHP installation properly”
this is due to hhvm settings, got /etc/hhvm/php.ini and comment out all the php options like so:
; php options
;session.save_handler = files
;session.save_path = /var/lib/php5
;session.gc_maxlifetime = 1440
restart hhvm and it should work.