GENWiki

function WriteFile($path,$filename,$content) {
    $file = JPATH_ROOT.'/'.$path."/".$filename;
    // the write coding
    JFile::write($file, $content);
}

This simple Joomla function leverages the framework to write a file to disk.

Use it like this

WriteFile('media','my_file.txt','This will be written to the file');

Which will write a file called my_file.txt into the folder ./media within your Joomla installation. You'll need to make sure the location is writeable.