code error using if in function wordpress?

how to correct this because after registration complete only generate BA.015 how to generate other???

add_action('user_register', 'my_on_user_register');
function my_on_user_register($user_id, $studi){
if ($studi = 'something') {
    $unique = "BA.015.";
} elseif ($studi = 'something1') {
    $unique = "PA.015.";
} else {
        $unique = "FA.007.";
};
            $args = array(
        'length'    => 2,
                'before'        => $unique
    );
    $random_number = my_random_string($args);
    update_user_meta($user_id, 'random_number', $random_number);

}