No, it is not difficult.
First thing you are going to need is a storage method.
I recommend MySQL, most decent hosts that offer PHP offer MySQL as well.
Other options; XML/Files, CSV Files, etc
For the table itself, a link and stylesheet with width and height set to 100% on the link. The link text can say whatever. If you are using a graphic background on your table, use a in the link for the text.
The link will be the driver, it will need to contain cell informaiton.
Example: <a href="reserve.php?r=3&c=2">
Of course, your reserve.php will have to make sure that the selected box is available before saving, as users could just change the URL in an attempt to take over a previously reserved spot.
That should get you started, research VIA google (search for exact text):
PHP and MySQL Example
PHP $_GET usage
* PHP Link Navigation
Good Luck