$compile_dir
     This is the name of the directory where compiled templates are
     located. By default this is
     "./templates_c"
     , meaning that it
     will look for the compile directory in the same directory as
     the executing php script. This directory must
     be writeable by the web server
     (see install).
     Also
     $use_sub_dirs.
    
Technical Note: 
       This setting must be either a relative or
       absolute path. include_path is not used for writing files.
      
Technical Note: 
       It is not recommended to put this directory under
       the web server document root.
      
     See also $compile_id
     and
     $use_sub_dirs.