The @simshaun dont work for me. I check the documentation and I found base_convert that can work for you (on base 35) and a comment of "francesco[at]paladinux.net" with a function to work on base65.
so the solution can be:
convert to b10 -> increment +1 -> convert base 65
EDIT 1
talking about convert I think to base64 coding so I wrote this 2 funcs using base64 encode/decode numbers.
Unfortunately the charset used is a bit larger: [a-zA-Z0-9/+=], but using an internal function more efficent.
The Zero
0 is "AA=="
function nencode($in){
$res ="";
while(1){
$b = $in & 0xff;
$res = chr($b) . $res;
if($in > 0xff){
$in = $in >> 8;
} else {
break;
}
}
return base64_encode($res);
}
function ndecode($in){
$rv=0;
$res =base64_decode($in);
for($t = 0 ; $t < strlen($res) ; $t++){
if($t>0){
$rv = $rv << 8;
}
$c = ord($res{$t});
$rv |= $c;
}
return $rv;
}