You may want to try setting the CURLOPT_INTERFACE
option:
curl_setopt($curlh, CURLOPT_INTERFACE, "xxx.xxx.xxx.xxx");
CURLOPT_INTERFACE: The name of the outgoing network interface to use. This can be an interface name, an IP address or a host name.
From: php Manual: curl_setopt
EDIT: Fixing example, as @Michael Hart pointed out pointed out in the other answer.