BEWARE, a couple of the examples in the comments suggest doing something like this:

chmod(file_or_dir_name, intval($mode, 8));

However, if $mode is an integer then intval( ) won't modify it.  So, this code...

$mode = 644;
chmod('/tmp/test', intval($mode, 8));

...produces permissions that look like this:

1--w----r-T

Instead, use octdec( ), like this:

chmod(file_or_dir_name, octdec($mode));

See also: http://www.php.net/manual/en/function.octdec.php

BEWARE using quotes around the second parameter...

If you use quotes eg

chmod (file, "0644");

php will not complain but will do an implicit conversion to an int before running chmod. Unfortunately the implicit conversion doesn't take into account the octal string so you end up with an integer version 644, which is 1204 octal

Usefull reference:

Value    Permission Level
400    Owner Read
200    Owner Write
100    Owner Execute
40    Group Read
20    Group Write
10    Group Execute
4    Global Read
2    Global Write
1    Global Execute

(taken from http://www.onlamp.com/pub/a/php/2003/02/06/php_foundations.html)

Windows has a very different file permission model to Unix and integrates them only minimally.

On Windows, all this function can do is to change the "read only" flag, which is turned on if $mode & 0200 does not pass.
i.e. it only checks if u+w is missing from the bitmask, and if it is, it sets the read only flag.

The executable flag cannot be set as Windows determines it based on file extension.
The write flag cannot be set as Windows determines write access based on ACLs, which are not integrated here.

If you cannot chmod files/directories with PHP because of safe_mode restrictions, but you can use FTP to chmod them, simply use PHP's FTP-functions (eg. ftp_chmod or ftp_site) instead. Not as efficient, but works.

if you want to chmod directories too, use this 

<?php
$iterator = new RecursiveIteratorIterator(new RecursiveDirectoryIterator($pathname), RecursiveIteratorIterator::SELF_FIRST);

foreach($iterator as $item) {
    chmod($item, $filemode);
}
?>