PHP Timing & ETC

Hello again.

Everyone knows some browser games and there's health, mana or some kind of resources that regenerates themselves automatically in some time. How I can do that in theory? What functions I must exploit to do such a feature? In example I can create a database table for that where I can store my health. I know how to decrease the amount and etc, but what is stable way to make health in database grow. Like increase it by 3 in one minute? I don't think that people will use cron jobs for that, because it will overflow whole server if it requires more execution time, because we have thousands of people to increase the value.

Well. I don't know how to explain it anymore, but I want it increasing with personal speed of player without taking a care about others. I don't want to do a simple script which increases everything. It must increase the HP with some kind of restoration speed like "3.0" (should be 3.0 per minute) if some player's restoration speed is 5.0 then it should be (5.0 per minute)

HP: 100/100 = No action
HP: 99/100 = Tick until 100
HP: <100 = Tick until 100

How the heck this is possible :confused:

Thanks.