I think it's no longer necessary to worry about trailing nulls. This was fixed in Feb 2002 (see bug 10133)

It may be worth noting that using gmp_strval can slow your script down a lot with large numbers.

This script for example, produces factorials in succession:
<?php

$start = microtime(TRUE);

$fact = gmp_init(1, 10);
for($i=1;$i<100000;$i++) { // calculates 100,000 factorials
  $fact = gmp_mul($fact, $i); // $fact is now the result of the $ith factorial
  // gmp_strval($fact, 10); // see below...
  if(microtime(TRUE)-$start>1) // stop calculating after 1 second
    break;
}
// print whatever result that took 1 second to complete
printf("\$i = %d\n\$fact = %s\n", gmp_strval($fact, 10));

?>

My server calculates around about the 69,500th factorial on average every time. Uncommenting that gmp_strval line slows this process down dramatically, and only calculates about 5,000 factorials. This is about 14 times longer than before (this is consistent with allowing it to run for any number of seconds, not just 1).

Nitrogen.

Base 2 ... 36 = digits and lowercase letters (a=10 ... z=35)
Base -2 ... -36 = digits and uppercase letters (A=10 ... Z=35)
Base 37 ... 62 = digits, then uppercase letters, then lowercase letters (A=10 ... Z=35, a=36 ... z=61)

A gotcha when using the value from gmp_strval as a key in an associative array. Some numbers are 2^31 are converted to signed integer format, this is usually incorrect.

<?php

$a = gmp_init('2147483649'); // = 2^31 + 1
$b = gmp_init('3173604585'); // = 2^31 < $b < 2^32 

$c = gmp_strval($a);
$d = gmp_strval($b);

$e = array($c => 'c', $d => 'd');

print '*** Expect '."\n";
print 'Array '."\n".'('."\n".'    ['.$c.'] => c'."\n".'    ['.$d.'] => d'."\n".')'."\n\n";
print '*** Actual '."\n";
print_r($e);

?>

Output:

*** Expect
Array
(
    [2147483649] => c
    [3173604585] => d
)

*** Actual
Array
(
    [-2147483647] => c
    [3173604585] => d
)

A workaround is to format as base 16 and append '0x' before using as a key.

Warning: gmp_strval may return a null terminated string (e.g., gmp_strval(gmp_init(8)) has two characters. This is because gmp is built for speed, so uses estimates based on things it can get quickly, like the number of bits in N. So since 8 is a 4 bit number, it will be allocated ceil(log10(2^4))=ceil(4*0.30103....) = 2 characters. The gmp folks feel it is the caller's responsibility (in this case the PHP interface) to check whether or not all the characters were required.  PHP might have done this for us (and may in the future), but for now, you need to check (if it matters to you--it sure did in my program!).

casting to string seems to do the same (for base10) and it's much faster

(string) $num

instead of 

gmp_strval($num)