First stable release of PHP version 5 was done two years ago. This version of language has matured a lot throughout these two years and with PHP version 4 development effectively stopped should already be considered the preferred platform by developers of new PHP libraries and applications.
PEAR policies encourage "backwards compatibility", which unfortunately means supporting the inadequate object model of PHP version 4. While it does make sense for existing packages, requiring such "backwards compatibility" for new packages that get accepted into PEAR has at least two problems:
It slows the adoption of PHP version 5 by users of PEAR
PEAR risks becoming a garbage dump of obsolescent code, or at least be perceived as one