Your wireless network is encompassed in a subnet with only one external IP. You could log it with
$_SERVER['REMOTE_ADDR']. Any machine in the subnet you're on will appear with that same IP. There is no way I know of to differentiate between machines on the same subnet but different wireless access points. I don't believe it should be possible, but the subnet is probably enough for you.
Beware that if the internet connection this subnet uses eventually gets a different IP address from the ISP (extremely likely), you will start seeing that different IP.
Hope that's enough for you to do whatever it is you're trying to do.