want to use a php variable as a session variable

i want to use a sssion variable like this

$var= $_SESSION['var1'];//this is a dynamic variable, can be altered along with each page refresh
$_SESSION['var']=0;
echo $var;//or like this echo $_SESSION['var'];

var is a php variable
i want to use that variable as session variable;
$_SESSION['$_SESSION['p_name']'];//is it possible?