<?php

$input = '1ab2v3g4de';
$length = \strlen($input);
$numerics = [];

for ($i = 0; $i < $length; ++$i) {
    if (\is_numeric($input[$i])) {
        $numerics[] = [$i => $input[$i]];
    }
}

$numLastIndex = \count($numerics) - 1;

foreach ($numerics as $key => $numericData) {
    $newKey = $numLastIndex - $key;
    $input[\key($numericData)] = $numerics[$newKey][\key($numerics[$newKey])];
}

\var_dump($input);