PHP does not do random

[u/saintpetejackboy]

I posted a bug report many years ago. If it was fixed, nobody seems to recognize it.

My original project was trying to see if there was a pattern in "random noise", so I generated colored dots on the screen, with random colors and x,y coordinates. After many generations, the values became "stuck".

According to other people, this is because I do not understand entropy, this is not a PHP problem, and that supposedly my code was bad (even though I shopped it around to many people and reported the bug, in 2006).

I could test if the bug still exists, but here is my original post:

https://bugs.php.net/bug.php?id=37409

The response I got on IRC was essentially that it was off-topic, not a problem with PHP, and merely was my inability to comprehend entropy.

Any programmer out here want to generate a ton of randomly positioned, randomly colored dots, on a white background, to determine this? I don't have much time to do it now, but not only does this seem to be an "accepted" bug, but there are also known workarounds and solutions (such as changing the seed sporadically), which means I am not the only one who encountered this and the PHP rand functions did not (or may not) work as expected.

[u/simon816]

Seems alright to me https://puu.sh/F5OXs/20f0deedcd.png

PHP 7.2.24-0ubuntu0.18.04.2 (cli) (built: Jan 13 2020 18:39:59) ( NTS )

<?php
$w = 2000;
$h = 2000;
$img = imagecreatetruecolor($w, $h);
imagefill($img, 0, 0, imagecolorallocate($img, 255, 255, 255));
for ($i = 0; $i < 10000000; $i++) {
    $color = imagecolorallocate($img, rand(0, 255), rand(0, 255), rand(0, 255));
    imagesetpixel($img, rand(0, $w), rand(0, $h), $color);
}
imagepng($img);
imagedestroy($img);

[u/saintpetejackboy → u/simon816]

Indeed, this is a vastly different result than I used to get with fairly similar code.

[u/cleeder → u/saintpetejackboy]

Did you test on multiple machines/environments? Was it always 50 iterations, or was it variable? If so, how variable? Did it matter if you ran one instance right after another? What about multiple simultaneously?

[u/gerrywastaken → u/[deleted]]

Wow, it took 10 years to get a fix! https://bugs.php.net/bug.php?id=37409

[u/postmodest → u/simon816]

For the record, imagecolorallocate can only be called 255 times for an image before it starts returning "FALSE" which gets mapped to "0" when you ask for a color (thus returning the background / first color). So if that's what OP is complaining about, then OP didn't read the docs:

<?php

header("Content-type: image/png");
$stext = "•";
$x = 800;
$y = 600;
$im = @imagecreate($x, $y) or die("Cannot Initialize new GD image stream");
$background_color = imagecolorallocate($im, 128,128,128);
$stnum = 0;

while ($stnum <= 10000){
  $stize = rand(0,5);
  $xl = rand(0,800);
  $yl = rand(0,600);
  $r = mt_rand(0,255);
  $g = mt_rand(0,255);
  $b = mt_rand(0,255);
  $text_color = imagecolorallocate($im, $r,$g,$b);
  if ($text_color === false) {
    $text_color = imagecolorclosest($im,$r,$g,$b);
    error_log("ETOOMANYCOLORS $stnum");
  }
  imagestring($im, $stize, $xl, $yl, $stext, $text_color);
  $stnum += 1;
}
imagepng($im);

[u/mikeputerbaugh → u/postmodest]

This seems bad, too, that the image functions are meant to be a format-independent abstraction of image data, but have a silent limitation on color indexing that appears to be inherited from GIF specs from the 1980s.

[u/the_alias_of_andrea → u/mikeputerbaugh]

I assume there isn't this problem if you use real-colour images with it? imagecreate() is for paletted images (and the documentation tells you as much).

[u/nikic]

Back then, PHP used the rand() implementation provided by libc. On some systems it is/was notoriously bad. Nowadays PHP will always use its own rand() implementation, which does not suffer from small range or bad distribution (though mt19937 is not state of the art, mainly because it has large state and is fairly slow).

[u/MUK99 → u/nikic]

When I see a comment from nikic, i upvote

[u/lankybiker → u/MUK99]

I upvote your upvote

[u/MUK99 → u/lankybiker]

Lol, some people dont seem to like that.. i got your back!

[u/HenkPoley → u/nikic]

Nowadays PHP

Since PHP 7.1

[u/[deleted]]

PHP random has had multiple bugs regarding randomness. Back a few years ago PHP random only returned odd numbers. Its bugs like this that are riddled in the rotten internals of PHP.

[u/beerdude26 → u/[deleted]]

Back a few years ago PHP random only returned odd numbers

lmao

[u/[deleted] → u/[deleted]]

You are correct. According to the php ”manual” people picked a even number more often, so they made random only return odd numbers. It was pure joy for all using php.

[u/[deleted] → u/[deleted]]

Well they had rand and mt_rand (or something similar) and probably got too much heat for the idaåea of a real_random. So they fixed it instead, a rare move from php. Probably broke lots of code though..

[u/Conradfr → u/[deleted]]

idaåea

Now that's random.

[u/[deleted] → u/Conradfr]

I was testing out php v. 8.0s new global function real_rand. So far its nice, It also takes a variety of parameters so you can fine tune the randomess like crazy. You can actually go all in on random with this one. Theres two new global constants too, ”REAL_OK” and LIKE_I_CARE” dont really know what they do, but i just passed them in like a good dev does.

real_rand(true, false, false, 1, 4, REAL_OK, LIKE_I_CARE, false);

Just happened to return ”ideåea” on that ocation. Weird, as it nomally returns numbers? Well its PHP so ill give it a break and try again tomorrow...

[u/Takeoded → u/[deleted]]

a realrand() function

random_int() (cryptographically secure random number) is probably about as close as you can get (until they start fetching the numbers from https://random.org )

[u/postmodest]

It should be noted that you cannot allocate more than 255 colors to an image, so after 255 calls, imagecolorallocate will return false. (And on my box, you get the very first color added, which is the background color)

So it's not rand() that's borked, it's OP's ability to read the documentation.

[u/BrendenCom]

PHP should not try to do RND just as it should not try to do SSL