.log文件并不总是通过ftp覆盖

I have a .log file I've been attempting to log to via php. Writing to it via php seems to be working sporadically for the same lines of code. In addition I also occasionally try to overwriting the file via ftp, this also only works sporadically. Can any one suggest what might be causing this behavior.

Here is a line of php demonstrating how I am writing to the file via php:

file_put_contents('aggregation5',"
test line",FILE_APPEND);

Update:

Everything works as expected if I don't use the .log extension. Is this an improper use of a .log file?