numbers.php

<?php 

function choices($start, $operation, $max = 999)
{
    $down = $operation < 0;

    $start = $down ? $start : $start + 1;
    $end = $start + ($down ? $operation : $operation - 1);

    $choices = [];
    foreach (range($start, $end) as $i) {
        $goBack = $i < 0 || $i > $max;
        $base = $down ? $max + 1 : 0;
        
        $choices[] = $goBack ? $base + $i : $i;
    }

    return $choices;
}


function get_interval_numbers($selected, $quantity, $max = 999) 
{
    return array_merge(
        choices($selected, $quantity, $max),
        choices($selected, $quantity * -1, $max),
    );
}


echo var_dump(get_interval_numbers(20, 50));