<?php

$string1 = '1: 2, 3';
$string2 = '1: 2, 3; 4: 5, 6';

function revert(string $string) {

$array = explode('; ', $string);

foreach ($array as $x => $entry) {

$subarray = explode(': ', $entry);

foreach ($subarray as $i => $subentry) {

$substring = explode(', ', $subentry);
$substringReverse = array_reverse($substring);
$subarray[$i] = implode(' ', $substringReverse);
}

$array[$x] = implode(': ', $subarray);
}

return implode('; ', $array);
}

$result = revert($string1);
print_r($result);