How to destroy an object?
You're looking for unset().
But take into account that you can't explicitly destroy an object.
It will stay there, however if you unset the object and your script pushes PHP to the memory limits the objects not needed will be garbage collected. I would go with unset() (as opposed to setting it to null) as it seems to have better performance (not tested but documented on one of the comments from the PHP official manual).
That said, do keep in mind that PHP always destroys the objects as soon as the page is served. So this should only be needed on really long loops and/or heavy intensive pages.
A handy post explaining several mis-understandings about this:
Don't Call The Destructor explicitly
This covers several misconceptions about how the destructor works. Calling it explicitly will not actually destroy your variable, according to the PHP5 doc:
PHP 5 introduces a destructor concept similar to that of otherobject-oriented languages, such as C++. The destructor method will becalled as soon as there are no other references to a particularobject, or in any order during the shutdown sequence.
The post above does state that setting the variable to null can work in some cases, as long as nothing else is pointing to the allocated memory.
Short answer: Both are needed.
I feel like the right answer was given but minimally. Yeah generally unset() is best for "speed", but if you want to reclaim memory immediately (at the cost of CPU) should want to use null.
Like others mentioned, setting to null doesn't mean everything is reclaimed, you can have shared memory (uncloned) objects that will prevent destruction of the object. Moreover, like others have said, you can't "destroy" the objects explicitly anyway so you shouldn't try to do it anyway.
You will need to figure out which is best for you. Also you can use __destruct() for an object which will be called on unset or null but it should be used carefully and like others said, never be called directly!
see:
http://www.stoimen.com/blog/2011/11/14/php-dont-call-the-destructor-explicitly/