There are several ways to do this pragmatically:
<?php
echo chr(163);
printf("%c", 163);
echo "£"; //preferred for HTML
?>
Another way to do this more "manually" would be to save your files in UTF-8 encoding.
See here for more information.
Here's a screenshot of the results:

As stated by primatology, if you're going to be ouputting to HTML, make sure to include the proper encoding header for HTML. This should be inserted between your <head>
tags.
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />