How is PHP able to run schedule tasks in such a way?

[Hall of Famer]

Well I remember there is a section in Vbulletin called schedule task in which admins simply create cronjobs through ACP. From my PHP experience, it is impossible to set up cronjobs without adding crons to server side through WHM/Cpanel. Now it seems that its possible to run cronjobs without consulting the hosting company. I wonder how Vbulletin is able to do that. I also noticed that a video sharing engine called Clipbucket has default cronjob sets too, so guess its not really a secret now?

[Zachery]

Everytime a vbulletin page is called, cron.php is also called and if a task needs to be run it is executed in the background.

[Lynne]

They are not "real" cron jobs. Real cron jobs running at a specific time set up on the server. These are just Scheduled Tasks that get trigger by users on the forum. If you don't have a very active forum, then the Scheduled Tasks may not run at exactly the time you set them to run at.

[Hall of Famer]

Ah I see. I am asking this question since I am designing an adoptables script. Some users want their pets to level up/gain experience every X hours instead of receiving clicks, but this definitely requires cronjob. I cant just tell them to consult their host company to set up cronjobs, these are absolutely newbies at PHP and HTTP server, they are gonna have a real hard time get a time-based event to work.

And btw, is it gonna increase server load and cause VB to run much slower if cron is called everytime a user accesses the forum? o_o

[Zachery]

If there is no task that needs to be ran, and no tasks in the backlog, no cron is run.