I am not sure why, and it could just be me but on my FreeBSD system using
$time = posix_times();
$time['ticks'] is an enormous value that bears no relation to the system uptime (I tested by rebooting the system, the number does not change).
I checked my timecounters, they tick every 10.000msec and I did the maths on the returned value and it suggested the machine had been up for over 200 days - it was reformatted about a week ago.
This could be to do with FreeBSD, or *BSD, or just *idiots like me but just check before you use the function.
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FreeBSD 5.1-R, Apache 2.0.46, PHP4.3.2