Displaying data for only the visible map area using php mapscript

[Arunoday Chatterjee]

I'm trying to develop this map with counties for a particular state, with
major highways and
secondary highways and streets. Streets is something that i want to
display when the map is
zoomed into to a certain level. Problem is that i'm not being able to load
street data for only the visible area of the map as a result of which
apache is failing to process as the
street data is enormously huge. Since it is just zooming i'm unable to get
any parameter to filter street data by.
Can anyone let me know how to do this, basically display data of a layer
for only the visible area in the map.
I'm using php mapscript 4.2.3.