<?php

function comb(string $input): array
{
    $pad = [
        2 => ['a', 'b', 'c'],
        3 => ['d', 'e', 'f'],
        4 => ['g', 'h', 'i'],
        5 => ['j', 'k', 'l'],
        6 => ['m', 'n', 'o'],
        7 => ['p', 'q', 'r', 's'],
        8 => ['t', 'u', 'v'],
        9 => ['w', 'x', 'y', 'z'],
        0 => [' '],
        1 => ['']

    ];

    $numbers = [];
    $arr     = array_map('intval', str_split(intval($input)));

    for ($i = 0; $i < count($arr); $i++) {

        $numbers [] = $pad[$arr[$i]];
    }

    return $numbers;
}

function combinations($numbers, $i = 0) : array
{
    // termination condition
    if (!isset($numbers[$i])) {
        return [];
    }

    // if we reached the last element of the array -> start next array
    if ($i == count($numbers) - 1) {
        return $numbers[$i];
    }

    // recursive call to get combinations from subsequent arrays
    $tmp = combinations($numbers, $i + 1);

    $result = [];

    // concat each array from tmp with each element from $numbers[$i]
    foreach ($numbers[$i] as $v) {
        foreach ($tmp as $t) {
            $result[] = is_array($t) ? array_merge([$v], $t) : [$v, $t];
        }
    }

    return $result;
}


function printResults(array $combinations)
{
    foreach ($combinations as $combination) {
        printf(implode('', $combination) . "\n");
    }
}

$numbers = comb('12345');

printResults(combinations($numbers));