By "get te page's source code"... I mean get the page's source code:
$source = file_get_contents( $url );
$source = md5( $source );
If the client writes anything in real-time to the page, it could wreak havoc on your plans. For instance, if they display the current time, users online, and so on. You may want to grab the source and, using a RegExp, grab a portion of the code you know will only be modified with major updates, but that will severely decrease the scripts ability to just plug in any old site and work as desired.
What you could also use is stream_get_meta_data():
$fp = fopen( $url, 'r' );
$data = stream_get_meta_data( $fp );
print_r( $data['wrapper_data'] );
fclose( $fp );
Will give you something like:
 => HTTP/1.1 200 OK
 => Date: Fri, 18 Jan 2008 15:31:42 GMT
 => Server: Apache/2.2.0 (Fedora)
 => Last-Modified: Sat, 23 Dec 2006 20:05:22 GMT
 => ETag: "9f251c-795f-15998480"
 => Accept-Ranges: bytes
 => Content-Length: 31071
 => Connection: close
 => Content-Type: text/html
You could then use the Last-Modified
header to check when the requested URL was... last modified.
Actually, if you visit http://www.php.net/stream_get_meta_data
, the current top-most comment (by ed at readinged dot com) is probably exactly what you're looking for.