Personally, I'd use file_put_contents and file_get_contents (these are wrappers for fopen, fputs, etc).
Also, if you are going to write any structured data, such as arrays, I suggest you serialize and unserialize the files contents.
$file = '/tmp/file';
$content = serialize($my_variable);
file_put_contents($file, $content);
$content = unserialize(file_get_contents($file));