How can I allow a PHP script to run once per day even if it is launched multiple times?
on a linux server use cron , you will be able to choose what frequency it will be run : https://help.ubuntu.com/community/CronHowto
on a windows server :
What is the Windows version of cron?
after your edit : see @script47's comment
By the way, cron is very simple to use, dont be affraid of it