Atypical letter and number incrementation
<p>I'm working on something which requires unique ID numbers which are in the format of:</p>

<blockquote>
  <p>[A-Z][A-Z0-9][A-Z0-9][A-Z0-9][A-Z0-9][A-Z0-9]</p>
</blockquote>

<p>And increment as:</p>

<blockquote>
  <p>AAAAAA, AAAAAB, ... AAAAAZ, AAAAA0, AAAAA1, .. AAAAA9, AAAABA, AAAABB</p>
</blockquote>

<p>I know I can increment letters in PHP, but how would I do letters and numbers in such a way?</p>

<p>Special note, baseconvert isn't an option, because it must always be exactly 6 characters and fit the noted format incrementally. Further, baseconvert starts at 0, not A, so if I do start at <strong>"621937810"</strong> (AAAAAA) the next jump will be after AAAAAZ and on to AAAAB0. It seemed like the quickest solution, but it doesn't work.</p>
