mt_srand

(PHP 4, PHP 5, PHP 7, PHP 8)

mt_srand — Seeds the Mersenne Twister Random Number Generator

Descripción

mt_srand(?int $seed = null, int $mode = MT_RAND_MT19937): void

Seeds the random number generator with seed or with a random value if no seed is given.

Nota: No es necesario usar una semilla para usar el generador de números aleatorios con srand() o mt_srand() ya que se hace automáticamente.

Precaución

Because the Mt19937 (“Mersenne Twister”) engine accepts only a single 32 bit integer as the seed, the number of possible random sequences is limited to just 2³² (i.e. 4,294,967,296), despite Mt19937’s huge period of 2¹⁹⁹³⁷-1.

When relying on either implicit or explicit random seeding, duplications will appear much earlier. Duplicated seeds are expected with 50% probability after less than 80,000 randomly generated seeds according to the birthday problem. A 10% probability of a duplicated seed happens after randomly generating roughly 30,000 seeds.

This makes Mt19937 unsuitable for applications where duplicated sequences must not happen with more than a negligible probability. If reproducible seeding is required, both the Random\Engine\Xoshiro256StarStar and Random\Engine\PcgOneseq128XslRr64 engines support much larger seeds that are unlikely to collide randomly. If reproducibility is not required, the Random\Engine\Secure engine provides cryptographically secure randomness.

Parámetros

seed

Fills the state with values generated with a linear congruential generator that was seeded with seed interpreted as an unsigned 32 bit integer.

If seed is omitted or null, a random unsigned 32-bit integer will be used.

mode

Use one of the following constants to specify the implementation of the algorithm to use.

MT_RAND_MT19937: The correct Mt19937 implementation, available as of PHP 7.1.0.
MT_RAND_PHP Uses an incorrect Mersenne Twister implementation which was used as the default up till PHP 7.1.0. This mode is available for backward compatibility.

Advertencia

This feature has been DEPRECATED as of PHP 8.3.0. Relying on this feature is highly discouraged.

Valores devueltos

No devuelve ningún valor.

Historial de cambios

Versión	Descripción
8.3.0	`seed` is now nullable.
7.1.0	srand() has been made an alias of mt_srand().
7.1.0	mt_rand() has been updated to use the fixed, correct, version of the Mersenne Twister algorithm. To fall back to the old behaviour, use mt_srand() with `MT_RAND_PHP` as the second parameter.

Ver también

mt_rand() - Generate a random value via the Mersenne Twister Random Number Generator
mt_getrandmax() - Show largest possible random value
srand() - Seed the random number generator

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User Contributed Notes 18 notes

down

e at juresah dot si ¶

4 years ago


Please note that according to:
https://stackoverflow.com/a/11358829/2897386

The seed is automatically initialised with the current timestamp if not provided.

This means that your script will produce values of poor random quality if it's always run at a predictable time, for example by crontab. In that case it may be a good idea to initialise it manually from a cryptographically secure source.

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Alderin1 at gmail dot com ¶

17 years ago


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.

down

slonmron_no_spam_please_ at yahoo dot com ¶

17 years ago


Looks like mt_rand() gives same result for different seeds when the lowest bits are different only. Try this:

#!/usr/bin/php -q
<?php

$min = -17;
$max = $min + 48; // 48 is to fit the results in my console

for ($testseed=$min; $testseed<$max; $testseed++)
{
    mt_srand( $testseed );
    $r = mt_rand();
    printf("mt_srand( 0x%08x ): mt_rand() == 0x%08x == %d\n", $testseed, $r, $r);
}

?>

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.

down

mrcheezy at hotmail dot com ¶

21 years ago


Very good points above on seeds, thank you. If you would like to test a seed try using the code below. It will take between 5 and 20 seconds depending on your system and then will spit out the number of reused keys out of 100,000 attempts.

;  for ($i=0; $i<100000; $i++) {
;    mt_srand(hexdec(substr(md5(microtime()), -8)) & 0x7fffffff);
;    $rand = mt_rand();
;
;    ($arr[$rand] == '1') ? $k++ : $arr[$rand] = '1';
;  }

down

Nibbels / downfight.de ¶

10 years ago


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<10;$i++){
    echo mt_rand();
}

echo "<BR>";

mt_srand(10); //start of your algorithm equals seeding set back to 10
for($i=0;$i<10;$i++){
    echo mt_rand();
}

Output is like:
502355954641584702211262118810740890731360749216120791137454651988317865160461082451610903986200
<BR>
502355954641584702211262118810740890731360749216120791137454651988317865160461082451610903986200

My conclusion: Don't preset your seed to the same number all the time if you want "alternating random numbers"

Greetings

down

dev at 10e12 dot net ¶

14 years ago


What about this for an example... 

(sorry for the funky line breaks but I have once
more reported the bug on this issue of word 
wrapping to no avail)

Intending to use it in passing a "semi-guaranteed"
properly seeded random number to a client, then to 
capture input from the user which must be encrypted 
client side before being sent to the server again 
  a) during the same session and, 
  b) within a set time limit.

for more reading also see:
  AES Rijndael enc/dec routines for javaScript
  developed and tested by Herbert Hanewinkel, 
  http://www.hanewin.net/encrypt/aes/aes.htm

<?php
/*
calling the function, prepping the data to send to 
the client... , who then uses it in a javascript 
implementation of AES Encrypt.
*/

function SHA256($str, $keyval=""){
    if ($keyval!==""){//not null or empty
        $sHash = mhash(Constant('MHASH_SHA256'),$str, $keyval);
    }else{
        $sHash = mhash(Constant('MHASH_SHA256'),$str);
    }
    // same as Binary 2 Hex conversion
    return implode(unpack('H*',$sHash),'');
}

/*---------------------------------------------------
Radomizing the random data with fixed data for 
the user and location If return value maximum 
is set to zero (0) the function uses mt_getrandmax
If both retMin and retMax is zero (0) the function 
uses mt_rand w.o. limits
*----------------------------------------------------*/
function local_prgn($retMin = 0, $retMax = 0){
// first get the session id of the request session
  $sSrv = session_id(); 

// the following is valid only on the current server
  $sSrv = implode(unpack($_SERVER['SERVER_NAME'].
             $_SERVER['SERVER_ADDR'].$sSrv),'');

// the following applies only to the requesting client
  $sReq = implode(unpack($_SERVER['REMOTE_ADDR'].
             $_SERVER['REQUEST_TIME']),'');

// get a SHA256 seed based on the current values
  $sSeed = SHA256($sSrv,$sReq); 

// get a random value based on the uniqueness of the seed above
    mt_srand($sSeed); // seed the random num gen

// some error handling and checking
    if ($retMin > $retMax){ 
    // swap vars if wrong order
        $rx = $retMax; $retMax = $retMin; $retMin = $rx; 

    } else if ($retMin == $retMax){
    //meaningless range, not very random
    //ensure for next check in this range
        $retMin = 0; $retMax = 0;
    }
// select what we need to return
    if ((($retMax == 0)&&($retMax = mt_getrandmax))||
($retMin==$retMax)){
        return mt_getrandmax();
    }else{
        return mt_getrandmax($retMin,$retMax);
    }
}

?>

down

Kapr ¶

17 years ago


To slonmron:
Seed for random numbers generator should be initialized only once, before calling proper rand function. After that you give pseudorandom sequence by multiple calling rand. Initialization of random seed is used if 1) You have better source of random seed than implemented algorithm or 2) if You need always the same sequence of pseudorandom numbers. Example given by You shows only that first rand result strongly depends on seed, what is by definition. It is not a bug.

down

vbassassin at coderheaven dot com ¶

18 years ago


"Better still: Use a 31-bit hash of microtime() as the seed. "

Correct me if i am wrong, but woudlnt using microtime() still limit the total seeds to 1,000,000 again? Since the 31-bit hash will always give the same hash for the same number, and in the microtime() function you could have 1,000,000 or less numbers. So in effect your still no better off at all :-p

Best regards,
scott

PS: I actually agree that PHP has pretty much resolved the issue and got as close as anyones going to get to solving the seeding issue by introducing the "Mersenne Twister" algorithm which creates a much larger pool than 1,000,000 numbers. Just because the mt_srand() function exists doesnt mean you HAVE to use it ;-) use it if you NEED a specific list of the same numbers (comes in handy for encryptions with passwords ;-)

down

maxim at php dot net ¶

22 years ago


to : l_rossato@libero.it

doing ...

list($usec,$sec)=explode(" ",microtime());
$unique = mt_srand($sec * $usec);

theoretiaclly, makes just as much sense as 

list($usec,$sec)=explode(" ",microtime());
$unique = $usec + 0;

Once every while, depending on the microsecond resolution of your computer, the millisecond value will be a zero (0), and as I hope you know, in mathematics, any number multiplied by a zero becomes a zero itself.

(x * 0 = 0)

In real life, on a good machine, with a resolution to 1 million miliseconds per each second (i.e: Win2k server), you will be reduplicating your unique ID each million's ID issued. This means if you use it as your cookie encryption algorithm or a visitor ID, you will not exceed some million instances.

Futhermore, if that would be for a software development that you re-distribuite, installed on some weird old PC, where resolution can be as small as 100 milliseconds per second - a code with this uniqueness algorithm just wouldn't last any long.

Good Luck,

Maxim Maletsky
maxim@php.net

PHPBeginner.com

down

changminyang at hananet dot net ¶

22 years ago


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

/* Test: Each get rand sequence are 10time. */
/* ex) 5.3point meaning 5point integer + 3point decimal */

// case A:
// 5.0point - 1time
// 6.0point - 9time
$rand = (double)microtime()*1000000;

// case B:
// 8.6point - 1time
// 9.4point - 1time
// 9.5point - 7time
// 10.3point - 1time
$rand = (double)$sec * $usec;

// My case A:
// 8.0point - 10time
$rand = explode(".",$usec * $sec);
$rand = (double)substr($rand[0]*$rand[1],0,8);

// My case B:
// 9.0point - 9time
// 10.0point - 1time
$rand = explode(".",$usec * $sec);
$rand = $rand[0] + $rand[1];

mt_srand($rand);
srand($rand);

// P.S> My previous note is has wrong lines, sorry about it.  This is right.

down

-1

dev at 10e12 dot net ¶

14 years ago


Sorry for the error in the previous...
Due to the glitch with the wordwrap I got annoyed and lost focus on the copy and paste move.

The last part of the actual function should read.

<?php
// select what we need to return
    if ((($retMax == 0)&&($retMax = mt_getrandmax))||
($retMin==$retMax)){
        return mt_rand();
    }else{
        return mt_rand($retMin,$retMax);
    }

?>

And Nothing else of course...

down

-1

php dot net-comment at lucb1e dot com ¶

10 years ago


@  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.

down

-1

fasaxc at yahoo dot com ¶

21 years ago


The best way to ensure a random seed is to do the following:
To start:
   1) get your initial seed with mt_srand(microtime() * 1000000)
   2) generate a random no. $random=mt_rand()
   3) save this number in a file (or database or whatever so that it is available next time the page is loaded)

 Now, for each time your script is loaded :
   1) load the value you saved above and do $new_seed=($random+(microtime() * 1000000))%pow(2,32)
   2) mt_srand($new_seed);
   3) generate a new random no. $random=mt_rand()
   4) save that number back in the file/database

This procedure takes advantage not only of the randomness of microtime() but of all the previous calls to microtime() so your seed becomes better and better with time. It also generates good seeds even on platforms where microtime() doesn't take all the values it can.

Just using microtime() * 1000000 only results in 1000000 possible seeds (and less on some platforms as noted) - the function above gives 2^32 seeds with an avelanche effect accross multiple executions.

down

-1

josh at joshstrike dot com ¶

13 years ago


mt_srand effectively performs a modulo % 2147483648 on positive integers over 32 bits, but with negative integers it instead adds 2147483648 to the value it gets.

Seeds with equal results:
2147483649 == 1
2147483648 == 0
2147483647 == -1 
-2147483646 == 2
-2147483647 == 1
-2147483648 == 0

Importantly though, seeding with anything less than -2147483648 will always yield the same result as seeding with zero.

down

-1

fasaxc at yahoo dot co dot uk ¶

21 years ago


In fact, here's an even better function than the one below assuming your install provides a random entropy daemon and you're running *nix (to check for the former type "head -c 6 /dev/urandom" on the command line if available - if you get 6 random characters you're set). N.B. php must be able to find the head program so it must be in your path and allowed if you're running safe mode.

The functions db_set_global() and db_get_global() I use to set/get a variable from a central database but you could save/restore the variable from a file instead or just use the function get_random_word().

<?

####################################
## returns a random 32bit integer.
## Passing a parameter of True gives a better random
## number but relies on the /dev/random device
## which can block for a long time while it gathers
## enough random data ie. DONT USE IT unless
##   a) You have an entropy generator attatched to
## your computer set to /dev/random -OR-
##   b) Your script is running locally and generating
## a good random number is very important
####################################
function get_random_word($force_random=False) {
    if ($force_random) {
        $u='';
    } else {
        $u='u';
    }
    $ran_string=shell_exec("head -c 4 /dev/{$u}random");
    $random=ord(substr($ran_string,0,1))<<24 |
            ord(substr($ran_string,1,1))<<16 |
            ord(substr($ran_string,2,1))<<8 |
            ord(substr($ran_string,3,1));
    return $random;
}

--EITHER - IF YOU'VE SET UP A DATABASE OF GLOBAL VARIABLES--

## If the seed is found in the database
if ($seed=db_get_global('seed')) {
    # use mt_rand() to get the next seed
    mt_srand($seed);
    # then XOR that with a random word
    $seed=(mt_rand() ^ get_random_word());
} else {
## Make a completely new seed (First Run)
    # Generate the seed as a proper random no using /dev/random
    $seed=get_random_word(True);
    mt_srand($seed);
}

db_set_global('seed',$seed);

--OR JUST--
mt_srand(get_random_word());

?>

down

-2

limo at anime42 dot com ¶

14 years ago


I have spent the last couple of hours trying to track down a bug which affects mt_rand/rand and mt_srand/mt_rand.

OS is Debian 5.0.4 "Lenny".
PHP version is 5.3.2-0.dotdeb.1 with Suhosin-Patch (cli) (built: Mar  9 2010 11:42:01).

I have tried to fix the issue by appending the following lines into the .htaccess / apache2 main config file:

        php_value suhosin.mt_srand.ignore Off
        php_value suhosin.srand.ignore Off

This has helped a bit, stabilizing the beggining of the pseudo random number sequence, but the generator still fails after a fair number of iterations (roughly around 1K~3K.

*** Removing the Suhosin extension has resolved this issue, I am waiting for an official extension build that will work with 5.3.x so that I can reattach it into the php configuration. ***

Here is the code which ought to replicate the problem:

    $len = 100000;
    $min = 0;
    $max = 99;

    $t = (int)(microtime(true)*0xFFFF);

    $a = array();
    srand( $t );

    for ( $i = 0; $i < $len; $i ++ )
        $a[$i] = rand( $min, $max );

    $b = array();
    srand( $t );

    for ( $i = 0; $i < $len; $i ++ )
        $b[$i] = rand( $min, $max );

    for ( $i = 0; $i < $len; $i ++ )
        if ( $a[$i] !== $b[$i] )
            die( 'Pseudo-random sequence borked at #'.$i.'th iteration!');

    echo 'Your pseudo-random sequencer is working correctly.';
    exit( 0 );

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-5

simon at labs dot coop ¶

10 years ago


I can’t stress how important it is to seed your randomisation process in code! better still something we found in the BBS Days was if we didn’t seed from a token from outside our systems abstraction layer we would go in circles and so would our users. Here at chronolabs we offer a feed of randomly changing token on each impression, it also randomly displays a different number of them this is from http://seed.feeds.labs.coop in the example below I use DOM to load the XML, Extract the randomisation tokens and then with mt_srand and srand seed the random selecting processes! The following function when you call it will seed your random selection process in both the old and new random selection routines all you need to do is call the function! This will work with any version of PHP 5 and any earlier with DOM Objectivity.

function makeRandomSeeded() {
    $file = 'http://seed.feeds.labs.coop/';
    $doc = new DOMDocument();
    $doc->loadHTMLFile($file);
    $skip = array('This feed can', 'Current mode is');
    $elements = $doc->getElementsByTagName('description');
    foreach($elements as $element) {
        $seed = $element->nodeValue;
        $found = false;
        foreach($skip as $find) {
            if (substr($seed, 0, strlen($find))==$find) {
                $found = true;
            }
        }
        if ($found==false)
            $seeds[] = $seed;
    }
    shuffle($seeds);
    mt_srand($seeds[mt_rand(0, count($seeds)-1)]);
    srand($seeds[mt_rand(0, count($seeds)-1)]);
}

Remember when PHP says an integer this also include any character of the Ascii chart if you would like to see an example of this do the following:

<?php
   $a = "000A";
   while($a!="001B") {
      echo $a;
      $a++;
   }
?>

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-3

oxai NOSPAM post dot htnet dot hr ¶

18 years ago


try this instead(!):

<?php

// randomizes MT's seed once per process.

function randomizeProcessSeed()
{
    static $thisProcessHasBeenInitialized;

    if( $thisProcessHasBeenInitialized ) return;

    list($usec, $sec) = explode(' ', microtime());
    mt_srand( (10000000000 * (float)$usec) ^ (float)$sec );
    
    $thisProcessHasBeenInitialized = true;
} randomizeProcessSeed();
?>

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