I made a function that can be used for converting numbers to any base you wish.. instead of returning a string of pre-defined index of characters (i.e. 0-9a-z) you could simply make your own of any length using the array of indexes it returns.
I looked around and didn't see anybody made one, I needed one for a simple compression algorithm with only numbers, I've not actually made it yet but this was an initial idea.

<?php
// ConvertBase function explained:
// we add an array item $Input%$Base floored and divide $Input by $Base floored.
// repeat until $Input is no longer above 0.

function ConvertBase($Input,$Base=10) {
  $Input=gmp_init($Input);
  $Result=array();

  for($i=0;$i<1||gmp_sign($Input)==1;$i++) {
    $Result[]=gmp_intval(gmp_mod($Input,$Base));
    $Input=gmp_div_q($Input,$Base);
  }
  $Result=array_reverse($Result);
  return($Result);
}

// an example how gmp_strval($.., 36); could be achieved:

// the funny emergency number from The IT Crowd
// (leading zeroes aren't liked in gmp_init though)
$Input = '1189998819991197253';

// our example 36 characters used in gmp_strval($.., 36);
$Chars = '0123456789abcdefghijklmnopqrstuvwxyz';

// count the $Chars so they're all used
// or use your own number less than the length of $Chars
$Base = strlen($Chars);

// perform
$Result = ConvertBase($Input,$Base);

// replace the resulting index with the corrosponding characters of the index in $Chars
for($i=0;$i<count($Result);$i++)
  $Result[$i]=$Chars{$Result[$i]};

// compare
printf("gmp_strval:  %s\r\n",gmp_strval($Input,36));
printf("BaseConvert: %s\r\n",implode($Result));

/* OUTPUT:
gmp_strval:  91h7dixfq6h1
BaseConvert: 91h7dixfq6h1
*/
?>
The example shows a familiar result of course, but the idea of this function was so that you can use whatever base you wish, and display entirely your own output to represent any number of choice.

Also, for those who wish to do bitwise shifting, it's quite simple.. to shift left, just multiply the number by pow(2,x), and to shift right, divide by pow(2,x).

<?php
function gmp_shiftl($x,$n) { // shift left
  return(gmp_mul($x,gmp_pow(2,$n)));
}

function gmp_shiftr($x,$n) { // shift right
  return(gmp_div($x,gmp_pow(2,$n)));
}
?>

Have fun,
Nitrogen.

For those (like me) who are trying to do bit masking with very large numbers, here is a useful function to do the work for you.

<?php
  function isBitSet($bitMask, $bitMap)
  {
    return (bool) gmp_intval(gmp_div(gmp_and($bitMask, $bitMap),$bitMask));
  }
?>