geo midpoint

function midpoint ($lat1, $lng1, $lat2, $lng2) {
        $lat1= deg2rad($lat1);
        $lng1= deg2rad($lng1);
        $lat2= deg2rad($lat2);
        $lng2= deg2rad($lng2);

        $dlng = $lng2 - $lng1;
        $Bx = cos($lat2) * cos($dlng);
        $By = cos($lat2) * sin($dlng);
        $lat3 = atan2( sin($lat1)+sin($lat2),
        sqrt((cos($lat1)+$Bx)*(cos($lat1)+$Bx) + $By*$By ));
        $lng3 = $lng1 + atan2($By, (cos($lat1) + $Bx));
        $pi = pi();
                return array('lng'=>($lat3*180)/$pi , 'lat'=>($lng3*180)/$pi);
    }