[imp] Can view attachment but cannot download

[Eric Tam]

Whenever I try to download attachment by pressing the download button, 
the following error messages appear.

Warning: Constant imp_base already defined in imp/view.php on line 12
Warning: Cannot add header information - headers already sent by (output 
started at imp/view.php:12) in imp/lib/Browser.php on line 68
Warning: Cannot add header information - headers already sent by (output 
started at imp/view.php:12) in imp/lib/Browser.php on line 76

My platform is:
Horde: 2.2.1
IMP: 3.2
PHP: 4.1.2
Linux: Redhat 7.2

Any suggestion?

Rgds
Eric Tam