tohop.php

<?php 
/**
* 
*/
class ToHop
{
	public $a = array();
	public $n;
	public $k;

	function __construct($n, $k)
	{
		$this->a[0] = 0;
		$this->n = $n;
		$this->k = $k;
	}

	function run()
	{
		$this->backtrack(1);
	}

	function backtrack($i)
	{
		for ($j = $this->a[$i - 1] + 1; $j <= $this->n - $this->k + $i; $j++)
		{
			$this->a[$i] = $j;
			($i == $this->k) ? $this->display() : $this->backtrack($i + 1);
		}
	}

	function display()
	{
		for ($j = 1; $j <= $this->k; $j++)
		{
			printf("%3d", $this->a[$j]);
		}
		echo "\n";
	}
}

$th = new ToHop(8, 5);
$th->run();