<?php

$id = strval(10001);
echo $id . PHP_EOL;

$id = str_pad($id, 8, '0', STR_PAD_LEFT);
echo $id . PHP_EOL;

$char_map = range('A', 'J'); // array(0 => A, 1 =>B, ..., 9 => J);
print_r($char_map);

$id = strtr(substr($id, 0, 4), $char_map) . substr($id, 4);
echo $id . PHP_EOL;

10001
00010001
Array
(
    [0] => A
    [1] => B
    [2] => C
    [3] => D
    [4] => E
    [5] => F
    [6] => G
    [7] => H
    [8] => I
    [9] => J
)
AAAB0001