help please

[spike spoke spoken]

hallo. I'm testing imp and when i ask for imp/index.php3 i'm getting the following message:

  Fatal error: Call to undefined               function:page_open() in /var/www/html/horde/imp/mailbox.php3 on line 104


the function is :
Page_open(array('sess'=>'HordeSession'));
 I'm using php-4.0.4 whith apache 1.3.19 in  red-hat 7.1 





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