how to store the time,date when we switch on our system?

[Arunincy]

Hi everyone,
I want to store the date&time in server database when a user switch on a system(ie)log in to the system.How can I do this in php?
Anyone Please help me

[Dormilich]

when a user switch on a system(ie)log in to the system

can you elaborate that? I have no idea what that means.