The mapping of values to characters occurs in a different order in
"standard" Base64 to the way base_convert() currently works, so
extending base_convert() out to 64 wouldn't make much sense, since it
would either be inconsistent with the existing behaviour of
base_convert() or base64_encode(), and either way I can only see it
confusing people.

Going to 62 would be possible, but as a cursory Web search for Base62
shows, there's no real standard -- even an informal one -- for how the
character set is ordered: the most common order seems to be 0-9a-zA-Z,
but there are plenty of examples of 0-9A-Za-z as well, not to mention
implementations that are simple variants of Base64 (which is, of
course, A-Za-z0-9, then two non-alphanumeric characters that nobody
seems to be able to agree on).

While it would be nice to have a Base62 conversion function (since it
is quite useful in URLs), I don't think the PHP standard library is
the place for it at the moment; developers will be better off using an
implementation that implements the actual variant they want.

Closing won't fix.