A function to get all pairs of array elements (配列要素の全てのペアを取得する関数)

pairs.php

<?php

/**
 * returns all pairs
 * @param   array   $items
 * @return  array
 */
function pairs(array $items)
{
    if (count($items) < 2) {
        throw new \Exception("Too few elements.");
    }
    $pairs = [];
    $lastIndex = count($items) - 1;
    for ($x = 0; $x < $lastIndex; $x++) {
        for ($y = $x + 1; $y <= $lastIndex; $y++) {
            $pairs[] = [$items[$x], $items[$y]];
        }
    }
    return $pairs;
}

$items = ['A', 'B', 'C', 'D', 'E'];
$pairs = pairs($items);
array_map(function ($pair) {
    echo implode(', ', $pair) . "\n";
}, $pairs);

This code results in:

$ php -f pairs.php 
A, B
A, C
A, D
A, E
B, C
B, D
B, E
C, D
C, E
D, E