Convert an arbitrarily large number from any base to any base.

convBase.php

<?php 

/*
Convert an arbitrarily large number from any base to any base.
 
string convBase(string $numberInput, string $fromBaseInput, string $toBaseInput)
 $numberInput number to convert as a string
 $fromBaseInput base of the number to convert as a string
 $toBaseInput base the number should be converted to as a string
 examples for $fromBaseInput and $toBaseInput
 '0123456789ABCDEF' for Hexadecimal (Base16)
 '0123456789' for Decimal (Base10)
 '01234567' for Octal (Base8)
 '01' for Binary (Base2) 
You can really put in whatever you want and the first character is the 0.
 Examples:
*/

convBase('123', '0123456789', '01234567'); 
//Convert '123' from decimal (base10) to octal (base8).
 //result: 173
 
convBase('70B1D707EAC2EDF4C6389F440C7294B51FFF57BB', '0123456789ABCDEF', '01');
 //Convert '70B1D707EAC2EDF4C6389F440C7294B51FFF57BB' from hexadecimal (base16) to binary (base2).
 //result: 
//111000010110001110101110000011111101010110000101110
 //110111110100110001100011100010011111010001000000110
 //001110010100101001011010100011111111111110101011110
 //111011
 
convBase('1324523453243154324542341524315432113200203012', '012345', '0123456789ABCDEF');
 //Convert '1324523453243154324542341524315432113200203012' from senary (base6) to hexadecimal (base16).
 //result: 1F9881BAD10454A8C23A838EF00F50
 
convBase('355927353784509896715106760','0123456789','Christopher');
 //Convert '355927353784509896715106760' from decimal (base10) to undecimal (base11) using "Christopher" as the numbers.
 //result: iihtspiphoeCrCeshhorsrrtrh
 
convBase('1C238Ab97132aAC84B72','0123456789aAbBcCdD', '~!@#$%^&*()');
 //Convert'1C238Ab97132aAC84B72' from octodecimal (base18) using '0123456789aAbBcCdD' as the numbers to undecimal (base11) using '~!@#$%^&*()' as the numbers.
 //result: !%~!!*&!~^!!&(&!~^@#@@@&
 
function convBase($numberInput, $fromBaseInput, $toBaseInput)
 {
     if ($fromBaseInput==$toBaseInput) return $numberInput;
     $fromBase = str_split($fromBaseInput,1);
     $toBase = str_split($toBaseInput,1);
     $number = str_split($numberInput,1);
     $fromLen=strlen($fromBaseInput);
     $toLen=strlen($toBaseInput);
     $numberLen=strlen($numberInput);
     $retval='';
     if ($toBaseInput == '0123456789')
     {
         $retval=0;
         for ($i = 1;$i <= $numberLen; $i++)
             $retval = bcadd($retval, bcmul(array_search($number[$i-1], $fromBase),bcpow($fromLen,$numberLen-$i)));
         return $retval;
     }
     if ($fromBaseInput != '0123456789')
         $base10=convBase($numberInput, $fromBaseInput, '0123456789');
     else
         $base10 = $numberInput;
     if ($base10<strlen($toBaseInput))
         return $toBase[$base10];
     while($base10 != '0')
     {
         $retval = $toBase[bcmod($base10,$toLen)].$retval;
         $base10 = bcdiv($base10,$toLen,0);
     }
     return $retval;
 }
 ?>

unfortunately it requires the extension bcmath :(