If you're using php's fwrite then according to the docs you don't need to flock when appending.
http://us3.php.net/manual/en/function.fwrite.php
Note:
If handle was fopen()ed in append mode, fwrite()s are atomic (unless the size of string exceeds the filesystem's block size, on some platforms, and as long as the file is on a local filesystem). That is, there is no need to flock() a resource before calling fwrite(); all of the data will be written without interruption.