Using $$ to access a variable

Please forgive my probably newbie question... I am cycling thru my database using a SQL describe statement and then am getting values from a form based upon the field name I find. This is working up 'til I try to read the form data. I have defined a variable like this: $var = $prefix.$table_array[$iCnt][0].$postfix; If I echo $var the printed result is: HTTP_POST_VARS["Id_val"] This is perfect because I want to access a field on the submitted form named "Id_val", but when I try echoing with a $$ like this: echo $$var I get nothing. I was under the impression that I could access my variable this way. Am I missing something basic here?? Thanks! Mark