dio_read timeout?

I'm using dio_read to read data from a serial port via a php cli script, this works great. However I'm unable to get dio_read to timeout if there's no data to read. No matter how I formulate the while loop, set_time_out, etc I can't seem to get it to stop reading and move on. Anyone have a work around or ideas? Example snippet, where $fd is a valid file descriptor. function readData($fd) {

        $startTime=microtime();
        $endTime=$startTime+'.02';

        $data=null;

$i=0;

        #while($startTime < $endTime){

while($i < 5) {

                $data.=dio_read($fd);

                if($data) {
                   echo "Here's some data: $data\n";
                   break;
                }

$i++;

        }

} //end function