How to create a match table

Question

array["Real Madrid","Barcelona","Atletic","Sevilla"]

and create, example

1
Real Madrid x Bacelona
Atletic x Sevilla
2
Real Madrid x Atletic
Sevilla x Barcelona
3
Sevilla x Real Madrid
Barcelona x Atletic

no repeat games, how make this?

https://en.m.wikipedia.org/wiki/Round-robin_tournament

Answer 1

$a = ["Real Madrid","Barcelona","Atletic","Sevilla"];
$length = count($a);
$counter = 1;
for ($i = 0; $i < $length - 1; $i++) {
for ($j = $i + 1; $j < $length; $j++) {
echo $counter++ . ' ';
echo $a[$i] . ' x ' . $a[$j];
echo "<br>";
}
}