[imp] Error in my PEAR using imp when login

Fri Oct 15 20:08:01 PDT 2004

Hi, 

Can any body help with imp or horde application...

I'm configuring Horde and Imp..   I have been set the setting of PHP and
Module of PHP.. 
PEAR is been updated.. but when I login in my IMP site...error occur 

The Error is: Fatal error: Call to undefined function: is_a() in
/usr/share/pear/PEAR.php on line 244

Could somebody help with this error

skeet