pdo binding not giving value - php

Question

the :cat_id has a value of nine and :tlk_id has a value of 3 if i enter them manually it will run with desired result but after binding them it gives me a row count of zero
<?
$sql = ("
SELECT
p.post_id as sn,
p.subject,
p.username,
COUNT(thread_id)-1 AS responses,
MAX(DATE_FORMAT(p.date_posted,'%e-%b-%y %l: %i %p')) AS first,
MAX(DATE_FORMAT(t.date_posted,'%e-%b-%y %l:%i %p')) AS last
FROM
posts AS p
INNER JOIN
threads AS t
USING
(post_id)
INNER JOIN
register AS u
ON
p.username = u.username
WHERE
t.category_id =:cat_id
AND
p.category_id =:cat_id
AND
t.forum_id = :tlk_id
AND
p.forum_id = :tlk_id
GROUP BY
(t.post_id)
ORDER BY
last DESC
LIMIT $start,$display
");
$sth = $dbh->prepare($sql);
$sth->bindParam(':tlk_id',$tlk_id);
$sth->bindParam(':cat_id',$cat_id);
$r = $sth->execute();
$count = $sth->rowCount();
echo $count;
?>

Answer 1

From the error:
Invalid parameter number: parameter ...
You can see that not all the parameters are processed. So maybe you can add 4 parameters in this case:
$sql = "SELECT
.....
WHERE
t.category_id =:cat_id_1
AND
p.category_id =:cat_id_2
AND
t.forum_id = :tlk_id_1
AND
p.forum_id = :tlk_id_2
....
";
$sth = $dbh->prepare($sql);
$sth->bindParam(':tlk_id_1',$tlk_id);
$sth->bindParam(':tlk_id_2',$tlk_id);
$sth->bindParam(':cat_id_1',$cat_id);
$sth->bindParam(':cat_id_2',$cat_id);

Answer 2

What happens if you do this:
$tlk_id = 3;
$cat_id = 9;
$sth = $dbh->prepare($sql);
$sth->bindParam(':tlk_id',$tlk_id);
$sth->bindParam(':cat_id',$cat_id);
I'm assigning the values to a variable because bindParam expects a reference as a second parameter.