Why do you want to use a different interface in each case? Unless you are benchmarking the interfaces and for some reason want to do that in php, then there is probably a better solution.
libcurl does implement this sort of functionality, but I'm not sure about php's binding for it. If it's not there, you could just use the curl
command directly via a shell call. From the man page:
--interface <name>
Perform an operation using a specified interface. You can enter
interface name, IP address or host name. An example could look
like:
curl --interface eth0:1 http://www.netscape.com/
(Wow. netscape.com, eh? Noone's gone over that man page in a while).
php script.php --interface=eth0 && php script.php --interface=eth1
etc.