Use type hinting in PHP bindings

[Olly Betts]

PHP 5 supports "type hinting", which we could use in a number of places in the PHP bindings:

​http://php.net/manual/en/language.oop5.typehinting.php

This would allow us to annotate the PHP method definitions to specify the type of object parameters (since 5.0) or that a parameter must be an array (since 5.1). There aren't many places where we accept arrays, so the latter isn't so useful, though for Xapian 1.3 we're only supporting PHP 5.2 and later anyway.

5.4 will add a "callable" type hint, which isn't so useful to us either.

Cases which are overloaded to take different object types would probably only get an Object hint, as it looks like only a single class can be specified. I'm not sure we have many of those.

If the argument has a default value of NULL, then NULL can be passed too. This would be needed in a few places, though I think most if not all already have a default value of NULL.

[Olly Betts]

Added these (via a script to post-process the wrapper script which SWIG produces) in r16203.