linux uptime history

Install uptimed. It does exactly what you want.

Edit:

You can apparantly include it in a PHP page as easily as this:

<? system("/usr/local/bin/uprecords -a -B"); ?>

Examples

the last command will give you the reboot times of the system. You could take the difference between each successive reboot and that should give the uptime of the machine.

update

1800 INFORMATION answer is a better solution.