Round robin algorithm in PHP

/******************************************************************************
* Round Robin Pairing Generator
* Author: Eugene Wee
* Date: 23 May 2005
* Last updated: 13 May 2007
* Based on an algorithm by Tibor Simko.
*
* Copyright (c) 2005, 2007 Eugene Wee
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
******************************************************************************/
function generateRoundRobinPairings($num_teams, $first_team='first', $second_team='second') {
    // Do we have a positive number of players? If not, default to 4.
    $num_teams = ($num_teams > 0) ? (int)$num_teams : 4;
    // If necessary, round up number of players to nearest even number.
    $num_teams += $num_teams % 2;
 
    // Format for pretty alignment of pairings across rounds.
    $format = "%0" . ceil(log10($num_teams)) . "d";
    $pairing = "$format-$format ";
 
    $games = array();
 
    // Generate the pairings for each round.
    for ($round = 1; $round < $num_teams; $round++) {
        $players_done = array();
        // Pair each player except the last.
        for ($player = 1; $player < $num_teams; $player++) {
            if (!in_array($player, $players_done)) {
                // Select opponent.
                $opponent = $round - $player;
                $opponent += ($opponent < 0) ? $num_teams : 1;
                // Ensure opponent is not the current player.
                if ($opponent != $player) {
                    // Choose colours.
                    if (($player + $opponent) % 2 == 0 xor $player < $opponent) {
                        // Player plays white.
                      $games[$round][$player][$first_team] = $player;
                      $games[$round][$player][$second_team] = $opponent;
                    } else {
                        // Player plays black.
                      $games[$round][$player][$first_team] = $opponent;
                      $games[$round][$player][$second_team] = $player;
                    }
 
                    // This pair of players are done for this round.
                    $players_done[] = $player;
                    $players_done[] = $opponent;
                }
            }
        }
 
        // Pair the last player.
        if ($round % 2 == 0) {
          $opponent = ($round + $num_teams) / 2;
          // Last player plays white.
          $games[$round][$player][$first_team] = $num_teams;
          $games[$round][$player][$second_team] = $opponent;
        } else {
          $opponent = ($round + 1) / 2;
          // Last player plays black.
          $games[$round][$player][$first_team] = $opponent;
          $games[$round][$player][$second_team] = $num_teams;
        }
    }
    return $games;
}