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(PHP 4 >= 4.0.4, PHP 5, PHP 7)
gmp_init — Create GMP number
$number 
   [,  int $base  = 0 
  ] )Creates a GMP number from an integer or string.
number An integer or a string. The string representation can be decimal, hexadecimal or octal.
base The base.
The base may vary from 2 to 36. If base is 0 (default value), the actual base is determined from the leading characters: if the first two characters are 0x or 0X, hexadecimal is assumed, otherwise if the first character is "0", octal is assumed, otherwise decimal is assumed.
GMP 数值 资源 .
| 版本 | 说明 | 
|---|---|
| 5.3.2 | The basewas extended from 2 to 36, to
        2 to 62 and -2 to -36. | 
Note:
To use the extended base introduced in PHP 5.3.2, then PHP must be compiled against GMP 4.2.0 or greater.
Example #1 Creating GMP number
  <?php
$a  =  gmp_init ( 123456 );
 $b  =  gmp_init ( "0xFFFFDEBACDFEDF7200" );
 ?>   Note:
It is not necessary to call this function if you want to use integer or string in place of GMP number in GMP functions, like gmp_add() . Function arguments are automatically converted to GMP numbers, if such conversion is possible and needed, using the same rules as gmp_init() .
[#1] marcus at synchromedia dot co dot uk [2011-08-11 04:41:01]
I discovered that the gmp functions use [0-9a-f] up to base 16, but [0-9A-Za-z] (i.e. upper case first) from bases 17 to 62. This differs from most of the base-62 implementations I've found that tend to use lower case first.
[#2] karl dot debisschop at pearson dot com [2011-03-29 12:15:46]
Unless the base is 16, gpm_init will fail if the string begins with "0b". 
> php -r '$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("$v\n");'
Resource id #4
[#3] charlie at oblivion dot cz [2007-02-18 04:55:25]
gmp_* functions don't accept strings with a leading '+':
<?php
echo gmp_strval(gmp_init('+42'));      #0
echo gmp_strval(gmp_add('42', '+42')); #42
echo bcadd('+42', '+42');              #84
?>
[#4] php at richardneill dot org [2006-09-20 03:30:50]
Here's a way to parse a decimal (eg 3.25) into an integer and exponent:
<?php
if (preg_match("/^[0-9]+\.[0-9]+$/",$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.
}
?>
[#5] php at richardneill dot org [2006-09-20 03:14:00]
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 $a.
[#6] thomas dot hebinck at digionline dot de [2005-08-18 10:23:06]
If you call a gmp_* function directly with an interger as parameter, this integer MUST NOT be 0:
for($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! :-)