mt_srand()

(PHP 4, PHP 5, PHP 7)

播下一个更好的随机数发生器种子

说明

用$seed来给随机数发生器播种。没有设定$seed参数时，会被设为随时数。

Note:自 PHP 4.2.0 起，不再需要用srand()或mt_srand()给随机数发生器播种，因为现在是由系统自动完成的。

参数

可选的种子值

返回值

没有返回值。

更新日志

范例

Example #1mt_srand()例子

参见

• mt_rand() 生成更好的随机数
• mt_getrandmax() 显示随机数的最大可能值
• srand() 播下随机数发生器种子

I think Joe was a little confused by the wording. The note meant that implementations of mt_rand() before the change would generate a different set of pseudorandom numbers than would implementations of mt_rand() after the change for the same seed.
That's how it reads for me, anyway.

Looks like mt_rand() gives same result for different seeds when the lowest bits are different only. Try this:
#!/usr/bin/php -q

This is a snapshop of the results:
...
mt_srand( 0xfffffffc ): mt_rand() == 0x0a223d97 == 170016151
mt_srand( 0xfffffffd ): mt_rand() == 0x0a223d97 == 170016151
mt_srand( 0xfffffffe ): mt_rand() == 0x350a9509 == 889885961
mt_srand( 0xffffffff ): mt_rand() == 0x350a9509 == 889885961
mt_srand( 0x00000000 ): mt_rand() == 0x71228443 == 1898087491
mt_srand( 0x00000001 ): mt_rand() == 0x71228443 == 1898087491
mt_srand( 0x00000002 ): mt_rand() == 0x4e0a2cdd == 1309289693
mt_srand( 0x00000003 ): mt_rand() == 0x4e0a2cdd == 1309289693
...
I found this occationally. I have no idea if it is a bug or not. In my real life I do not intend to use sequentional seeds. However, probably this may be important for somebody.

used the little script from mrcheezy at hotmail dot com and got much better results using
mt_srand(crc32(microtime()));

@ fasaxc at yahoo dot com:
If you want truly random numbers, use a truly random source. Your system is rather unwieldy when you can simply call openssl_random_pseudo_bytes() for good randomness. Don't use microtime as a source of randomness.

If you are new with seeding read my note.
I now understood seeding as a start-state of an algorithm. This algorithm generates a series of -following- pseudorandom numbers.
If you start generating from the same startvalue twice, you get the same series of random numbers twice in a row.
mt_srand(10); //start of your algorithm equals seeding set to 10
for($i=0;$i

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