Find other 2 vertices of GPS rectangle area

Question

In my PHP project I have the GPS coordinates of two opposite vertices of a rectangle (topleft and bottomright). I need to locate the other two.

Since this is in the standard GPS coordinate system the sites are parallel to the axis. I don't know if the geodetic vs planar base counts. I guess this is easy but my math knowledge is not the best.

Answer 1

We'll name the two coordinates x1,y1 and x2,y2 (where x is along, and y is up)

Therefore:

Top left: x1, y1
Top right: x2, y1
Bottom left: x1, y2
Bottom right: x2, y2