This would need to be client-side, ruling out PHP.
With Javascript, that'd be doable. Simply split the page into 2 frames; 1 of the frames can be hidden (either using CSS styles or making the width like.. 1 pixel, etc. CSS styling would be better... maybe "display:none", etc.?) and give the frame an ID.
Then,
document.getElementById('myFrame').onload = document.getElementById('myFrame').location = "page2.htm"
or something like that.
NOTE: This solution is Javascript, not PHP.