<?php
class processor {
	function _callback($m, &$ids) {
		$len = strlen($m[1]);
		if (isset($ids[$len])) ++$ids[$len]; else $ids[$len] = 'a';
		return '<'.$len.$ids[$len].'>';
	}

	function process($str) {
		return preg_replace_callback('/\*(\**)/',
			create_function('$m', 'static $ids=array(); return processor::_callback($m,$ids);'),
			$str)."\n";
	}
}

$proc = new processor;
echo $proc->process('******* *** *** ***** ***** ***');
echo $proc->process('***** *** *** ***** ***** ***');
echo $proc->process('******* *** *** ***** ******* ***');