// poprawny wynik
$query=sprintf("
    SELECT
        *
    FROM
        %1\$s
    WHERE
        (id_user = '%2\$d' AND id_zebra != '%2\$d') OR (id_zebra = '%2\$d' AND id_user != '%2\$d')
    AND
         active = '1'",
    TABLE_USERS_FRIENDS,
    $_SESSION['user']['id']
);

// drugie zapytanie - wynik błędny
$query=sprintf("
    SELECT
        *
    FROM
        %1\$s
    WHERE
    active = '1'
    AND
        (id_user = '%2\$d' AND id_zebra != '%2\$d') OR (id_zebra = '%2\$d' AND id_user != '%2\$d')",
    TABLE_USERS_FRIENDS,
    $_SESSION['user']['id']
);