This function is a recursive iterator function that gets all "unique" combinations with the given items.

subset-sum.php

<?php

function subsetSumRecursive($numbers, $arraySize, $level = 1, $i = 0, $addThis = [])
{
    if ($level == $arraySize) {
        $result = [];
        for (; $i < count($numbers); $i++) {
            $result[] = array_merge($addThis, array($numbers[$i]));
        }
        return $result;
    }

    $result = [];
    $nextLevel = $level + 1;
    for (; $i < count($numbers); $i++) {
        $newAdd = array_merge($addThis, array($numbers[$i]));
        $temp = subsetSumRecursive($numbers, $arraySize, $nextLevel, $i, $newAdd);
        $result = array_merge($result, $temp);
    }

    return $result;
}

$numbers = [1, 2, 3, 4, 5, 6, 7];

echo "<pre>";
print_r(subsetSumRecursive($numbers, 7));
echo "</pre>";