PHP doesn't come with a sendfax() function. You need to have some setup which can send faxes, and then write your own function to talk to that setup.
Come to think of it, if you were in a room without a fax machine or equivalent, you couldn't send a fax either.
In order for software send a fax, you need access to the phone network and a fax driver. You also need some sort of an interface to the driver (some way of asking the driver to send your fax for you).
If you have all of the above, then you can get PHP to send faxes for you, but you will need the documentation for the fax interface in order to see how to call it.
If you don't have all of the above, and can't get them in place, there may be some online service which you can use which will provide it for you - but I would imagine you will need to pay for it. Then the providers of that service will give you the documentation on how to call their service to send the fax.
