shuffler.example.php

<?php

function processPage($pdf, $page) {
	$pdf->addPage ();
	processSection ($pdf, $page['left'], 0);
	processSection ($pdf, $page['right'], 147.5);
}

foreach (preparePapers(41) as $paper) {
	processPage ($pdf, $paper['back']);
	processPage ($pdf, $paper['front']);
}

shuffler.php

<?php

// this class represents the sections.
class Section {
	var $pageNumber = false;
	var $nextSection;
}

// one paper has 4 sections. 2 at the back and 2 at the front
class Paper {
	var $backLeft;
	var $backRight;
	var $frontLeft;
	var $frontRight;
	function __construct() {
		$this->backLeft = new Section;
		$this->backRight = new Section;
		$this->frontLeft = new Section;
		$this->frontRight = new Section;
		$this->backRight->nextSection = $this->frontLeft;
		$this->frontLeft->nextSection = $this->frontRight;
		$this->frontRight->nextSection = $this->backLeft;
	}
	function toArray() {
		return array(
			'back' => array(
				'left'  => $this->backLeft->pageNumber,
				'right' => $this->backRight->pageNumber
			),
			'front' => array(
				'left'  => $this->frontLeft->pageNumber,
				'right' => $this->frontRight->pageNumber
			)
		);
	}
}

function preparePapers($numPages) {
	
	// calculate the number of requied paper.
	$numPaper = ceil($numPages / 4);
	$papers = array();

	// create papers and arrange them on top of the last paper.
	for ($i = 0; $i < $numPaper; $i ++) {
		$papers[$i] = new Paper;
		if ($i > 0) {
			$papers[$i - 1]->frontLeft->nextSection = $papers[$i]->backRight;
			$papers[$i]->backLeft->nextSection = $papers[$i - 1]->frontRight;
		}
	}

	// number each page
	for ($i = 1, $currentSection = $papers[0]->backRight;
			$i <= $numPages;
			$i ++, $currentSection = $currentSection->nextSection) {
		$currentSection->pageNumber = $i;
	}

	// flatten paper (clean array)
	foreach ($papers as $k => $v) {
		$papers[$k] = $v->toArray();
	}

	return $papers;

}