Result in decimals?

Result in decimals?

Alright so I've been trying to make two values in two different tables add
up to one number. Its for posting and comments how many a user has done.
The .$Author['id']. reads from another php file and is fine how it is.
When I use the code below it all works but the numbers are in decimals. So
if $result has a value of 4 and $result2 is 2 it would come up saying 0.6.
I have made sure that there is nothing wrong with the results or num_rows
by replacing the echo with echo "$num_rows"; and it comes up with the
right result. But why do when I try to add these together they become a
decimal?
$link = mysql_connect("--------", "---------", "--------");
mysql_select_db("--------", $link);
$result = mysql_query("SELECT * FROM post_threads WHERE author =
'".$Author['id']."'", $link);
$num_rows = mysql_num_rows($result);
$result2 = mysql_query("SELECT * FROM post_comments WHERE userid =
'".$Author['id']."'", $link);
$num_rows2 = mysql_num_rows($result2);
$a = array(".$num_rows." + ".$num_rows2.");
echo "" . array_sum($a) . "\n";
?>