How to put variable variable in SQL command in php?

Question

I want to fetch data from SQL. The code below is part of my code in php. My problem is that I don't know how to put $$start_year_col in sql command in php. every start1 or start2 are variables which consist year like 2015 or 2008 etc.

$start_year_col=start1    or it contains start2 



        $sql = "SELECT * FROM `t1`WHERE Market_indicator='STOCK' AND country='$Sel_Country' AND commodity='$Sel_Commodity' AND (month=$$start_month_col AND year = $$start_year_col)
        UNION
        SELECT * FROM `t1`WHERE Market_indicator='CROP' AND country='$Sel_Country' AND commodity='$Sel_Commodity' AND (month=$$start_month_col AND year = $$start_year_col)";