fwrite() - problems....

I have a large string that I want to written to a new file. Let's say the string contains the characters, "hello!" and I want the filename to be "hello.txt" Here is the code I have... $filename = "hello.txt"; $outputstring = "hello"; fwrite($filename, $outputstring); ...and of course it isn't working. I get this error... Warning: Supplied argument is not a valid File-Handle resource in /home/filanya/www/biopsy/export.php What am I doing wrong? ThANKS!!