超越PHP - Exceed PHP: mt_srand

说明

void mt_srand ( int seed)

用 seed 来给随机数发生器播种。


<?php

// seed with microseconds

function make_seed() {

    list($usec, $sec) = explode(' ', microtime());

    return (float) $sec + ((float) $usec * 100000);

}

mt_srand(make_seed());

$randval = mt_rand();

?>

注: 在 PHP 4.2.0 中，无需用函数 srand() 或 mt_srand()来搜寻随机数生成器，它将被自动完成。

参见 mt_rand()，mt_getrandmax() 和 srand()。

add a note User Contributed Notes
mt_srand

php_public at macfreek dot nl
07-Oct-2001 11:57

The seed is not good. The make_seed() function returns a big number (around
1.0E14), this number is bigger then 2147483647, the max. size of an
integer. If the result of make_seed is first converted to an integer, which
-on my setup- always becomes -2147483648. So, the above code always returns
the same seed and I always end up with the same random
numbers.



Note: This might be related to a bug noted on http://www.php.net/manual/en/language.types.integer.php.
But I couldn't confirm this, since it will be fixed in PHP 4.0.7, but that
isn't released yet at the time of writing)



I was also not able to
fix it by return the seed modulo 2147483647.

So for now, I just suggest
change the seed function as:



function make_seed() {

    return
(double)microtime()*1000000;

}



See also: uniqid()

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