How to copy a file from one directory to another using PHP?

You could use the copy() function :

// Will copy foo/test.php to bar/test.php
// overwritting it if necessary
copy('foo/test.php', 'bar/test.php');

Quoting a couple of relevant sentences from its manual page :

Makes a copy of the file source to dest.

If the destination file already exists, it will be overwritten.