I'm getting a problem with the PHP copy function. It fails when I use it to copy a 6Gb file.

-rw-r--r-- 1 root root 4294967296 2011-01-17 10:31 tape-17Jan2011-1025.tap
-rw-r--r-- 1 root root 6831937996 2011-01-17 10:25 tape.tap

The code:
PHP Code:
$backupFilename 'tape.tap';
$newName "tape-" date("dMY-Hi"filemtime($backupFilename));
if(!
copy($backupFilename"$newName.tap"))
{
    die(
"Error in copy operation");

but if i use exec to run 'cp' it works without errors:

-rw-r--r-- 1 root root 6843004548 2011-01-17 12:25 tape-17Jan2011-1201.tap
-rw-r--r-- 1 root root 6843004548 2011-01-17 12:01 tape.tap

PHP Code:
    exec("cp $backupFilename $newName.tap"); 
php -v
PHP 5.3.3-1ubuntu9.1 with Suhosin-Patch (cli) (built: Oct 15 2010 14:17:04)
Copyright (c) 1997-2009 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2010 Zend Technologies