WordPress is old. In fact, it is older than PHP7, in which PHP introduced random_int(). WP wanted/needed this functionality before, so another method was implemented.
how does the PHP interpreter understand which of the two functions I’m calling?
Good question. The interpreter doesn’t understand this. And hence, if you had PHP7 and would define this function new, you would get an error. This is why. the file with the function definition is only loaded, when random_int() is not available by default.
wp-includes/random_compat/random.php lines 212-214 are
if (!function_exists('random_int')) {
require_once $RandomCompatDIR.'/random_int.php';
}
-
So if your server is PHP7 and PHP’s own
random_int()is callable, this one is used and the file never included. -
If your server is not PHP7 or PHP’s own
random_int()is not callable for any reasons, the file is included and another implementation will be given.
This is done, so WordPress can run on different systems. Those with PHP5 and those with PHP7.
How is it different? I can’t really speak to it. The files WP uses seem to be from this random_compat repository, which is also linked from the PHP doc (and suggested if you don’t have PHP’s own implementation available).
One quick difference that I already saw: PHP’s random_int() tries to use getrandom(2) on Linux machines, while the compat random_int() only uses /dev/urandom.
When in doubt, I would use the system’s versions (PHP) instead of those introduced by software (WP). But that is only my opinion.