str_shuffle and randomness

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A better solution would be mt_rand which uses Mersenne Twister which much more better.

As has been pointed out, the str_shuffle method is not equivalent to the code I’m already using and will be less random due to the string’s characters remaining the same as the input, only with their order changed. However I’m still curious as to how the str_shuffle function randomizes its input string.

To make the output equal lets just use 0,1 and look at the visual representation of each of the functions

Simple Test Code 

header("Content-type: image/png");
$im = imagecreatetruecolor(512, 512) or die("Cannot Initialize new GD image stream");
$white = imagecolorallocate($im, 255, 255, 255);
for($y = 0; $y < 512; $y ++) {
    for($x = 0; $x < 512; $x ++) {
        if (testMTRand()) { //change each function here 
            imagesetpixel($im, $x, $y, $white);
        }
    }
}
imagepng($im);
imagedestroy($im);

function testMTRand() {
    return mt_rand(0, 1);
}

function testRand() {
    return rand(0, 1);
}

function testShuffle() {
    return substr(str_shuffle("01"), 0, 1);
}

Output testRand()

enter image description here

Output testShuffle()

enter image description here

Output testMTRand()

enter image description here

So basically I’d like to know how str_shuffle randomizes the string. Is it using rand() or mt_rand()? I’m using my random string function to generate passwords, so the quality of the randomness matters.

You can see clearly that str_shuffle produces almost same output as rand …

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