my solution was this:
--page /sendsms/?phone_nr=1122 sends one sms to phone nr 1122
--page /sendbulk has a loop which calls in php the page /sendsms/?phone_nr=1122 with file function file('http://www.domain.com//sendsms/?phone_nr=' . $phone_nr[$i]);
--the client on submit will call with ajax(no return needed) the page /sendbulk
This way even if the client closes the connection the server will continue to run /sendbulk in background and will send all messages