Description:
------------
Using the "If-Modified-Since" header is desirable when copying from a remote web server, to avoid unnecessarily copying unmodified files across the Internet.

However, the copy() function does not handle a HTTP 304 response, which indicates that the file on the web server is NOT newer than the local file. It ignores the response code, uses the (empty) content from the HTTP response and overwrites the existing local file with a zero length file. This is not a useful course of action.

Since the copy() function DOES handle other 3xx return codes appropriately (such as following permanent and temporary redirects), it would make particular sense to also handle 304.

Test script:
---------------
<?php
// $fURI: 		URL to a file located on a web server
// $target_file:	Path to a local file	

$arrRequestHeaders = array(
	'http'=>array(
		'method'		=>'GET',
		'protocol_version'	=>1.1,
		'follow_location'	=>1,
		'header'		=>'If-Modified-Since: '.date( 'r', filemtime( $target_file ) )."\r\n"
			)
		);
$rc = copy( $fURI, $target_file, stream_context_create($arrRequestHeaders) );
?>

Expected result:
----------------
When a HTTP 304 response is returned, the copy() function must NOT touch the target file. copy() could return a FALSE (indicating that nothing was copied), and the script could check the HTTP response to decide on any specialized processing.

Actual result:
--------------
When a HTTP 304 response is returned, the copy() function returns TRUE and overrides the target file with a 0 length file.