<?php
$players = range(1, 8);
 
echo 'Original List: ' . PHP_EOL;
print_r($players);
echo PHP_EOL;
echo 'Round Robin Schedule: ' . PHP_EOL;
print_r(generateRRSchedule($players));
?>

Original List: 
Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
    [4] => 5
    [5] => 6
    [6] => 7
    [7] => 8
)
 
Round Robin Schedule: 
Array
(
    [0] => Array
        (
            [0] => Array
                (
                    [0] => 1
                    [1] => 8
                )
 
            [1] => Array
                (
                    [0] => 2
                    [1] => 7
                )
 
            [2] => Array
                (
                    [0] => 3
                    [1] => 6
                )
 
            [3] => Array
                (
                    [0] => 4
                    [1] => 5
                )
 
        )
 
(...)
 
)

<?php
/**
 * Generate a round robin schedule from a list of players
 *
 * @param <array> $players	A list of players
 * @param <bool> $rand		Set TRUE to randomize the results
 * @return <array>			Array of matchups separated by sets
 */
function generateRRSchedule(array $players, $rand = false) {
	$numPlayers = count($players);
 
	// add a placeholder if the count is odd
	if($numPlayers%2) {
		$players[] = null;
		$numPlayers++;
	}
 
	// calculate the number of sets and matches per set
	$numSets = $numPlayers-1;
	$numMatches = $numPlayers/2;
 
	$matchups = array();
 
	// generate each set
	for($j = 0; $j < $numSets; $j++) {
		// break the list in half
		$halves = array_chunk($players, $numMatches);
		// reverse the order of one half
		$halves[1] = array_reverse($halves[1]);
		// generate each match in the set
		for($i = 0; $i < $numMatches; $i++) {
			// match each pair of elements
			$matchups[$j][$i][0] = $halves[0][$i];
			$matchups[$j][$i][1] = $halves[1][$i];
		}
		// remove the first player and store
		$first = array_shift($players);
		// move the second player to the end of the list
		$players[] = array_shift($players);
		// place the first item back in the first position
		array_unshift($players, $first);
	}
 
	// shuffle the results if desired
	if($rand) {
		foreach($matchups as &$match) {
			shuffle($match);
		}
		shuffle($matchups);
	}
 
	return $matchups;
}
?>