matrix.php

// generate sample of symmetry set (matrix)
$samlpes = [];
$items = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
foreach ($items as $k => $v) {
    foreach ($items as $k2 => $v2) {
        $samples[$k][$k2] = abs($v - $v2); 
    }
}

// init symmetric set storage
function getMatrixUniqID($x, $y) {
    if ($x >= $y) {
        return ($x * ($x + 1) / 2) + $y;
    } else {
        return ($y * ($y + 1) / 2) + $x;
    }
}
$items_count = count($samples);
$matrix_size = ($items_count * ($items_count + 1) / 2) + 1;
$computed = new \SplFixedArray($matrix_size);

// test
$x = 5;
$y = 6;
$xy = $matrix[getMatrixUniqID($x, $y)]; 
$yx = $matrix[getMatrixUniqID($y, $x)]; 
if ($xy == $yx) {
    echo 'hit';
}