The example given in the above comment is potentially confusing, since the 4th and 5th arguments appear to be the width and height of the link area, but are actually the coordinates of the diagonally opposite corner. This is much more evident when the starting point for the weblink is something other than 0,0 (which is of course very likely).
A better example:
$starting_xpos = 100;
$starting_ypos = 150;
pdf_add_weblink($pdf, $starting_xpos, $starting_ypos, $starting_xpos + $width, $starting_ypos + $height, "http://goweb.com.au/");