Mt_rand返回0

Question

mt_rand(~PHP_INT_MAX, PHP_INT_MAX);

Run a couple of times gives me the following output:

php > echo mt_rand(~PHP_INT_MAX, PHP_INT_MAX) . PHP_EOL;
0
php > echo mt_rand(~PHP_INT_MAX, PHP_INT_MAX) . PHP_EOL;
0
php > echo mt_rand(~PHP_INT_MAX, PHP_INT_MAX) . PHP_EOL;
-3445259641993297920
php > echo mt_rand(~PHP_INT_MAX, PHP_INT_MAX) . PHP_EOL;
0
php > echo mt_rand(~PHP_INT_MAX, PHP_INT_MAX) . PHP_EOL;
0
php > echo mt_rand(~PHP_INT_MAX, PHP_INT_MAX) . PHP_EOL;
0

It does not seem very random. Could anyone explain why it so often returns 0? I was expecting a "random" number between the lowest and highst available

Even more, if you change it abit like

echo mt_rand(-PHP_INT_MAX, PHP_INT_MAX)

It will start returning 1 instead of 0

Answer 1

It is always a good idea to read the manual when in doubt.

PHP - mt_rand

Caution The distribution of mt_rand() return values is biased towards even numbers on 64-bit builds of PHP when max is beyond 2^32. This is because if max is greater than the value returned by mt_getrandmax(), the output of the random number generator must be scaled up.

As mentioned in the comments by marcio the right solution would be

mt_rand(-mt_getrandmax(), mt_getrandmax())