I'm using the following libpdf versions:
PDFlib GmbH Version 6.0.1
PECL Version 2.0.3
I found the parameter 'top' to be misleading. Since 0,0 is in the bottom left of the page, the top parameter is actually the bottom left corner of the box you want.
using
top=72
left=72
puts the bottom left corner of the box 1 inch in from the left, and 1 inch up from the bottom.
so to make a 6.5"x3" box (with 1" page margins) at the BOTTOM of the page, you would use:
top=72
left=72
width=468
height=216
The text would then start in the top-left corner of the defined box, which would be at (72,288) (in 1" from the left and up 4" from the bottom)