Rough algorithm to calculate pages for a booklet

booklet_pages.php

function collateBookletPages($totalPages)
{
  // See if the page count is odd. If so, add 1 page to make it even
  if($totalPages % 2) {
    $totalPages++;
  }
  
  // $totalSides is a bit misleading, but couldn't think of a better term at the moment.
  // We are referring to one side of a sheet of paper (obverso/reverso) upon which
  // 2 pages will be printed on the left and right.
  $totalSides = $totalPages / 2;
  
  $side = []; // We will collect the pages in an array.
  for($x = 0; $x < $totalSides; $x++)
  {
    $sideIdx = $x + 1;
    if($sideIdx % 2) {
      // Working with an odd side
      $side[$sideIdx]["left"] = $totalPages - $x;
      $side[$sideIdx}["right"] = $x + 1;
    } else {
      // Working with an even side
      $side[$sideIdx]["left"] = $x + 1;
      $side[$sideIdx]["right"] = $totalPages - $x;
    }
  }
}

// The above just determines which page numbers will be printed where. You will need to work out the logic for getting the correct
// content onto each page.

// Here is a trace:
collateBookletPages(15);
$totalPages = 15 --> count is Odd so we add 1 --> $totalPages = 16
$totalSize = $totalPages / 2 = 16 / 2 = 8

$x | $sideIdx | "left" | "right"
 0 |     1    |   16   |    1
 1 |     2    |    2   |   15
 2 |     3    |   14   |    3
 3 |     4    |    4   |   13
 4 |     5    |   12   |    5
 5 |     6    |    6   |   11
 6 |     7    |   10   |    7
 7 |     8    |    8   |    9