Just use:
<?php
include("/path/to/file.html");
?>
That will echo it as well. This also has the benefit of executing any PHP in the file.
If you need to do anything with the contents, use file_get_contents(),
For example,
<?php
$pagecontents = file_get_contents("/path/to/file.html");
echo str_replace("Banana", "Pineapple", $pagecontents);
?>
This doesn't execute code in that file, so be careful if you expect that to work.
I usually use:
include($_SERVER['DOCUMENT_ROOT']."/path/to/file/as/in/url.html");
as then I can move files without breaking the includes.