Here's a way to parse a decimal (eg 3.25) into an integer and exponent:

<?
if (preg_match("/^[0-9]+\.[0-9]+$/",$input)){ 
     //Input is a base-10 decimal. Multiply as necessary to remove the decimal
     //point. Convert that to a gmp_resource, then decrement the exponent 
     //to compensate.

     $pieces=explode(".", $input);     //Split at the d.p.
     $input="$pieces[0]$pieces[1]";  //Remove the decimal point.

     $input=ltrim($input,'0');    
     //Remove any leading zeros, or gmp_init will parse the number as octal.

     if ($input==''){    //Deal with "0.0" which would otherwise be ''.
          $input=0;
      }
      $integer=gmp_init($input);   
      $ns_exponent=-strlen($pieces[1]);  
     //exponent = (-)  the number of characters after the decimal point.
}
?>

Unless the base is 16, gpm_init will fail if the string begins with "0b". 

> php -r '$v = gmp_init("b83", 17); print("$v\n");'

Resource id #4

> php -r '$v = gmp_init("0b83", 17); print("$v\n");'

[nothing prints]

In may case, where I am explicitly specifying the base, the solution is to apply ltrim first:

> php -r '$v = gmp_init(ltrim("0b83", "0"), 17); print("$v\n");'

Resource id #4

I discovered that the gmp functions use [0-9a-f] up to base 16, but [0-9A-Za-z] (i.e. upper case first) from bases 17 to 62. This differs from most of the base-62 implementations I've found that tend to use lower case first.

Missing from the documentation: gmp_init() return false in case it failed to parse $num

<?php
var_dump(gmp_init(""));
// returns "bool(false)" and not a GMP object as hinted by the documentation, it also triggers a Warning "PHP Warning:  gmp_init(): Unable to convert variable to GMP - string is not an integer in php shell code on line 1"

?>

gmp_* functions don't accept strings with a leading '+':
<?php
echo gmp_strval(gmp_init('+42'));      #0
echo gmp_strval(gmp_add('42', '+42')); #42
echo bcadd('+42', '+42');              #84
?>

Note: Leading zeros will make gmp_init parse this as octal.
Thus gmp_init(010) becomes 8.  

$a=010;              //8
$b="010" + 0;     //10
$c=gmp_strval(gmp_init(010));    //8
$d=gmp_strval(gmp_init("010")); //8

This behaviour is inconsistent: either $d should equal $b, or
$b should equal $a.

If you call a gmp_* function directly with an interger as parameter, this integer MUST NOT be 0:

for($i=-1;$i<=1;$i++) {  echo gmp_strval(gmp_add(2,gmp_mul(1,$i))) . ' ';  }

The result is 1 0 3 (wrong)

In this case you have to use gmp_init():

for($i=-1;$i<=1;$i++) {  echo gmp_strval(gmp_add(2,gmp_mul(1,gmp_init($i)))) . ' ';  }

The result is 1 2 3 (right)

Happy number crunching! :-)