How can i syn wordpress user table with a stand-alone php script?

WordPress use a special password hash then you need to send the password and the hash to the PHP code of WordPress to know if it’s valid.
This can be done with creating a new REST endpoint or with including the file “wp-includes/class-phpass.php” of WordPress and use it like this :

require_once "class-phpass.php";

$wp_hasher = new PasswordHash(8, TRUE);

$correct = $wp_hasher->CheckPassword($enteredPassword, $hashFromDatabse));
// $correct contains TRUE or FALSE