Пример:
$url="http://www.google.com/";
$ch=curl_init();
curl_setopt($ch,CURLOPT_URL,$url);
curl_setopt($ch,CURLOPT_INTERFACE,'212.0.12.45');
curl_setopt($ch,CURLOPT_RETURNTRANSFER,1);
curl_setopt($ch,CURLOPT_POST,0);
$content=curl_exec($ch);
curl_close($ch);