PHP 7.0.6 Released

    gmp_prob_prime

    (PHP 4 >= 4.0.4, PHP 5, PHP 7)

    gmp_prob_prime — Check if number is "probably prime"

    Description

    int gmp_prob_prime ( GMP $a [, int $reps = 10 ] )

    The function uses Miller-Rabin's probabilistic test to check if a number is a prime.

    Parameters

    a

    The number being checked as a prime.

    Either a GMP number resource in PHP 5.5 and earlier, a GMP object in PHP 5.6 and later, or a numeric string provided that it is possible to convert the latter to a number.

    reps

    Reasonable values of reps vary from 5 to 10 (default being 10); a higher value lowers the probability for a non-prime to pass as a "probable" prime.

    Either a GMP number resource in PHP 5.5 and earlier, a GMP object in PHP 5.6 and later, or a numeric string provided that it is possible to convert the latter to a number.

    Return Values

    If this function returns 0, a is definitely not prime. If it returns 1, then a is "probably" prime. If it returns 2, then a is surely prime.

    Examples

    Example #1 gmp_prob_prime() example

    <?php
    // definitely not a prime
    echo gmp_prob_prime("6") . "\n";

    // probably a prime
    echo gmp_prob_prime("1111111111111111111") . "\n";

    // definitely a prime
    echo gmp_prob_prime("11") . "\n";
    ?>

    The above example will output:

    0
1
2

    User Contributed Notes

    florin dot ciuica at yahoo dot com
    1 year ago
    <?php
        $max     = 2147483647;
       
            $primesFound = 0;
            $probablePrimes = 0;

        for (    $x = 1; $x <= $max; $x++) {
                $primeStatus = gmp_prob_prime($x);
            if (    $primeStatus == 1) {
                    $probablePrimes++;
            } else if (    $primeStatus == 2) {
                    $primesFound++;
            }
        }
        echo     "Total primes found: " . $primesFound . " between 1 and " . $max . ". Probable primes in this interval: " . $probablePrimes;
    ?>

    Based on that the following results were obtained:

    1 - 100000      - certain primes found: 9592,     probable: 0
    1 - 1000000     - certain primes found: 78498,    probable: 0
    1 - 10000000    - certain primes found: 78498,    probable: 586081
    1 - 100000000   - certain primes found: 78498,    probable: 5682957
    1 - 1000000000  - certain primes found: 78498,    probable: 50769036
    1 - 2147483647  - certain primes found: 78498,    probable: 105019067
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