All operators should have spaces around them ($den=null and $n/$d).
Return and pass-by-ref should be removed unless you provide a compelling reason for keeping it in. If for no other functions, for reduce and simplify this should be done as it may return the passed in object *or* a new one. This could quite easily cause confusion.
Single quotes when requiring PEAR.php
(Optional) I would rather $n.'/'.$d than "$n/$d"
(Optional) Remove the fold markers. IMHO we shouldn't have all of the extra text in our code for a small subset of editors which don't understand normal function folding semantics.
(Optional) If you keep the folding markers I would rather the end markers not be on the same line as }.
If keeping fold markers, the opening fold marker for a class should have the class name in it.