Used an array_reverse at the end so the initial round could work with seeding a tourney.

round-robin-algorithm.php

<?php

/**
  * Thanks https://en.wikipedia.org/wiki/Round-robin_tournament#Scheduling_algorithm
  *
  * @param int $weeks
  * @param array $teams
  * @return array
  */
function generateRoundRobin($rounds = 4, $teams)
{
    //Check if team count is odd
    if (count($teams) % 2 !== 0) {
        $teams[] = 'BYE';
    }
    
    $halfTeams = count($teams) / 2;

    $mTeams = $teams;
    
    array_shift($mTeams);
    
    $count = count($mTeams);
    
    $schedule = [];
    for ($i = 0; $i < $rounds; $i++) {
        
        $teamIndex = $i % $count;
    
        $schedule[$i][] = [$teams[0], $mTeams[$teamIndex]];
    
        for ($j = 1; $j < $halfTeams; $j++) {
            $firstTeam = ($i + $j) % $count;
            $secondTeam = ($i  + $count - $j) % $count;
            
            $schedule[$i][] = [$mTeams[$firstTeam], $mTeams[$secondTeam]];
        }
    }
    
    return array_reverse($schedule);
}

$rounds = 7;
$teams = [
    '1', '2', '3', '4', '5', '6', '7', '8'
];

$schedule = generateRoundRobin($rounds, $teams);

print_r($schedule);