Can I make a MySQL var equal to a MySQL command?

If I perform the following MySQL query through PHP:

UPDATE pictures 
        SET category = '0'
        WHERE category = '$categoryID'
        AND username = '$username'";
$queryUncat = mysql_query($uncategorise) or die(mysql_error());

It works fine and any category that was equal to $categoryID gets changed开发者_如何学Python to 0. However, if I perform the following:

UPDATE pictures 
        SET category = '0',
        pictureorder = (SELECT COUNT(category) + 1 WHERE category='0' AND username='$username')
        WHERE category = '$categoryID'
        AND username = '$username'";
$queryUncat = mysql_query($uncategorise) or die(mysql_error());

Not only does pictureorder not equal to the count of the category row plus one, but the category no longer gets changed if equal to $categoryID. I'm not too good at figuring this out as I know only basic MySQL through PHP and am not familiar with it through its own console.

Thanks in advance for any suggestions.

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This form of query is not valid. First, your subquery is missing a FROM clause. Second, you cannot select from the same table you are updating in the same query.

http://dev.mysql.com/doc/refman/5.1/en/update.html