How to declare and use a variable in BI Publisher report

Decalring the Varible :<?xdoxslt:set_variable($_XDOCTX, 'var', 0)?>

Do the calculation :<?xdoxslt:set_variable($_XDOCTX,'var', xdoxslt:get_variable($_XDOCTX,'var‘)+ XML columnname)?>

Display the Variable : <?xdoxslt:get_variable($_XDOCTX, 'var')?>