Cannot redeclare class mdb2_error in Pear MDB2 package

I haven't install pear in my PC,instead I download the packages and unzip them to "libs" dictionary in my web-app.the structure of dictionary of my web-app is 开发者_如何学JAVAroughly like this:

web-app
  +-/backend
    +-/libs
      +-/peardb
        +-pear.php
        +-pear5.php
        +-mdb2.php
        +-/mdb2

The application works fine in my two PCs,but throws "Cannot redeclare class mdb2_error" error while it running in virtual host.

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Using include_once / require_once can usually solve this problem.