file_put_contents problem

I have read in the contents of a file using file_get_contents. I can verify that the data has actually been read in by echoing its contents. But then if I do this: $ret = file_put_contents("/tmp/bla", $bk); The return value gives the correct size of string $bk, and the file /tmp/bla is created in /tmp, but the length is 0. Why are not the contents written to the file? -Andres