php configure options  ./configure   --enable-fpm   --enable-libxml   --with-mcrypt   --enable-mbstring   --with-gd   --with-mysql-sock   --with-mysqli=mysqlnd   --with-pdo-mysql=mysqlnd   --with-mysql=mysqlnd   --enable-sockets   --with-iconv   --with-gettext   --with-zlib   --with-freetype-dir=/usr   --with-jpeg-dir=/usr   --prefix=/usr/local/php-fpm   --with-config-file-path=/usr/local/php-fpm/etc   --with-config-file-scan-dir=/usr/local/php-fpm/etc/conf.d   --with-fpm-user=www-data   --with-fpm-group=www-data   --disable-simplexml   --disable-xmlreader   --disable-xmlwriter   --disable-tokenizer   --without-sqlite3   --without-pdo-sqlite   --with-curl

Sorry, but your problem does not imply a bug in PHP itself.  For a
list of more appropriate places to ask for help using PHP, please
visit http://www.php.net/support.php as this bug system is not the
appropriate forum for asking support questions.  Due to the volume
of reports we can not explain in detail here why your report is not
a bug.  The support channels will be able to provide an explanation
for you.

Thank you for your interest in PHP.

Ok. I try to reconstruct it other way

$query = "SELECT t.forum_id, count(topic_id) AS count_sticky FROM phpbb_topics as t, phpbb_prices_forums as f WHERE t.forum_id=f.forum_id AND f.forum_id IN (509, 545, 25, 45, 543, 20, 35, 487, 223, 288, 224, 256) AND topic_type=1 and topic_sticky_flag > 0 GROUP BY t.forum_id HAVING count_sticky <5";
$result = mysqli_query($link, $query) ;
print_r(mysqli_fetch_assoc($result));
print_r(mysqli_fetch_assoc($result));

gives:
Array
(
    [forum_id] => 256
    [count_sticky] => 4
)
Array
(
    [forum_id] => 543
    [count_sticky] => 1
)

this code works as well
$query = "SELECT t.forum_id, count(topic_id) AS count_sticky FROM phpbb_topics as t, phpbb_prices_forums as f WHERE t.forum_id=f.forum_id AND f.forum_id IN (509, 545, 25, 45, 543, 20, 35, 487, 223, 288, 224, 256) AND topic_type=1 and topic_sticky_flag > 0 GROUP BY t.forum_id HAVING count_sticky <5";
$result = mysqli_query($link, $query) ;
$row2 = mysqli_fetch_assoc($result);
print_r($row2);
$row2 = mysqli_fetch_assoc($result);
print_r($row2);

output:
Array
(
    [forum_id] => 256
    [count_sticky] => 4
)
Array
(
    [forum_id] => 543
    [count_sticky] => 1
)

BUT 

$query = "SELECT t.forum_id, count(topic_id) AS count_sticky FROM phpbb_topics as t, phpbb_prices_forums as f WHERE t.forum_id=f.forum_id AND f.forum_id IN (509, 545, 25, 45, 543, 20, 35, 487, 223, 288, 224, 256) AND topic_type=1 and topic_sticky_flag > 0 GROUP BY t.forum_id HAVING count_sticky <5";
$result = mysqli_query($link, $query) ;
echo "now ready to get row\n";
while($row2 = mysqli_fetch_assoc($result));
{
    echo "got row\n";
    print($row2);
    echo gettype($row2);
}
output is:

now ready to get row
got row
NULL

So here I used only php with no 3rd party sofware or functions. And this IS a PHP bug.

Hope this additional hode will make it clear. Thank you for your attention.

You've a dangling semi-colon in your while statement.

Thank you so much for your patience. That's really a silly mistake, that none of 3 programmers noticed and didn't change in their own tests. Thanks a lot.