Get all the combinations a number can be split into

I have a fixed number (5) and a fixed length (2):

I want to find all the combinations that can sum 50 (this can vary of course).

For example, for 50, and a length of 2, I want to get:

[[1, 49], [2, 48], [3, 47], [4, 46], ...],

For 50 and a length of 4, I want to get:

[[1, 1, 1, 47], [1, 1, 2, 46], [1, 1, 3, 45], [1, 1, 4, 44], ...],

I have no clue how to do this, or whether there's a name in mathematics for this.

This is what I have (doesn't have to use generators):

function getCombinations(array $numbers, int $sum, int $setLength) : \Generator {
    $numbersLowestFirst = $numbers;
    asort($numbersLowestFirst);
    $lowestNumber = 0;
    foreach ($numbersLowestFirst as $number) {
        // array might be associative and we want to preserve the keys
        $lowestNumber = $number;
        break;
    }

    $remainingAmount = $sum - $lowestNumber;
    $remainingNumberofItems = $setLength - 1;

    $possibleItemCombinations = array_pad([$remainingAmount], $remainingNumberofItems, $lowestNumber);

}