Working with large numbers in PHP

Question

To use modular exponentiation as you would require when using the Fermat Primality Test with large numbers (100,000+), it calls for some very large calculations.

When I multiply two large numbers (eg: 62574 and 62574) PHP seems to cast the result to a float. Getting the modulus value of that returns strange values.

$x = 62574 * 62574;
var_dump($x);          // float(3915505476) ... correct
var_dump($x % 104659); // int(-72945)  ... wtf.

Is there any way to make PHP perform these calculations properly? Alternatively, is there another method for finding modulus values that would work for large numbers?

Answer 1

For some reason, there are two standard libraries in PHP handling the arbitrary length/precision numbers: BC Math and GMP. I personally prefer GMP, as it's fresher and has richer API.

Based on GMP I've implemented Decimal2 class for storing and processing currency amounts (like USD 100.25). A lot of mod calculations there w/o any problems. Tested with very large numbers.

Answer 2

use this

 $num1 = "123456789012345678901234567890";
 $num2 = "9876543210";
 $r    = mysql_query("Select @sum:=$num1 + $num2");
 $sumR = mysql_fetch_row($r);
 $sum  = $sumR[0];

Answer 3

have you taken a look at bcmod()? php has issues with integers over 2^31 - 1 on 32 bit platforms.

var_dump(bcmod("$x", '104659') ); // string(4) "2968"