How to do multiply or similar with variables?

Hello

I am having difficulties getting the right result (basically I get the value "0") when trying to get the right value.

$result=mysql_query("SELECT ((Kind*$a_kind)+(Nice*$a_nice)+(Brave*$a_brave)+(Caring*$a_caring)) as total, Chars FROM char_game_chars GROUP BY Chars ORDER BY total ASC LIMIT 1")

From above code, the $a_kind,*$a_nice,*$a_brave and $a_caring depends on users answers, ans should get the "Chars" value that corresponds to the highest sum.

How would I create the right expression to calculate the right value?