I'm not sure PHP's chmod() will recursively reset all the files in a directory. When you issue the command you gave, I believe its just going to change the attributes of the directory, but not the files IN the directory.
It may be failing because your directory path is incorrect. Make sure off of the server's root directory/folder there is a directory/folder named "uploads". I'd predict your web host has something like /usr/home/[your account name]/public_html/[web site goes here]
and you've put "uploads" under [your account name]. But "/uploads/" references a directory found at root or on the same level as "usr".