How to use the IP address.
In order to view your IP address you can simply view the text file in a web browser by viewing the file as if it were a web page, e.g. http://iotha.co.uk/local_ip_address.txt
You can also use the embed tag to embed it in any HTML page to make it look a little better. Just ensure the HTML page is in the same folder as the text file or use a relative path.
<p>IP Address:</p>
<embed src="local_ip_address.txt">
If you want to access the value in a PHP file on your server you can read it into a variable.
You can then use it to send data to a port or read information from your local network.
$file = file_get_contents('http://iotha.co.uk/local_ip_address.txt');
You can see in the PHP webhook example here of how do this.
Security
The PHP script and examples here don't take security into consideration. You need to be careful about what you do with the script and how the IP address is used in order not to open your network to security threats.
I would recommend not sharing the PHP script location or password or your IP address with anyone else.
It would also be sensible to include some kind of authentication when sending or requesting information from your local network.