There's a really horrible bug (though they won't call it that!) in Apache's mod_rewrite that means that urlencoded inputs in rewrites get unescaped in their transformation to output patterns. The bug actually remains unfixed, though a workaround first appeared in Apache 2.2.12. [...] Base-62 is interesting as it can be made safe for use in URLs, DNS, email addresses and pathnames, unlike any available encoding of base-64, as it only includes [0-9A-Za-z].

He originally wrote his own parser, but notes that now the BCMath and gmp extensions make it much simpler, just a call to gmp_strval with gmp_init. This method works, but it's still not quite all he wanted so he created his own encoder to do the job.