The PEAR package contains:
* the PEAR installer, for creating, distributing and installing packages
* the alpha-quality PEAR_Exception php5-only exception class
* the beta-quality PEAR_ErrorStack advanced error handling mechanism
* the PEAR_Error error handling mechanism
* the OS_Guess class for retrieving info about the OS where PHP is
  running on
* the System class for quick handling common operations with files and 
  directories
* the PEAR base class

WWW: http://pear.php.net/package/PEAR/