[horde] error msg

[Yves bougardier]

Hi,
I'm a horde administrator, i tryed to install it on my Mandrake 8.2 with apache and sendmai and i have the following error when i want to access to my horde webpage :
Fatal error: Failed opening required 'PEAR.php' (include_path='') in /var/www/html/horde/lib/Registry.php on line 4

Can you help me ?
Sorry for this newbie question :-)) 
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