what are the different ways to use the variable in many functions within a module?
variable_set


  sets a persistent variable.
  
  Case-sensitivity of the variable_* functions depends on the database
  collation used. To avoid problems, always use lower case for
  persistent variable names.


variable_get


  Returns a persistent variable.
  
  Case-sensitivity of the variable_* functions depends on the database
  collation used. To avoid problems, always use lower case for
  persistent variable names.


variable_del


  Unsets a persistent variable.
  
  Case-sensitivity of the variable_* functions depends on the database
  collation used. To avoid problems, always use lower case for
  persistent variable names.