Please use ' instead of " for any strings which do not have backslash expressions (\n) or variables in them.
Please create and use a new Exception class for this package which extends PEAR_Exception (see Matthew's comment).
Please don't add try/catch blocks which only catch and re-throw exceptions. This is useless and confusing. (Such as in Parse.php __construct(), but I also saw this in most of the other files)
Please use true instead of TRUE.
I would prefer using ' for all strings except those with backslash expressions as variable interpolation is more expensive and less visible than concatenation.
Please use ++$var instead of $var++ unless you must use a post ++. Prepending it is slightly more efficient and is less prone to error.
The valid() function in Parser.php could be replaced with:
return $this->current_item < $this->feed->numberEntries;