php中唯一正变量的总和

I have variables $var1, $var2, $var3. Each day they are being fetched from a MYSQL query and being displayed in a row in a table. Sometimes they are positive and sometimes there are negative values. I would like one column to show the sum of only the POSITIVE values. How do I write a php statement to add only the positive values? Do I need to write 6 if statements or is there an easier way?. Each day