Using substr on a very large file

I have a very large file. I also have a memory limit on PHP and this file is larger than my memory limit.

What I would like to do is append a multiline string to this very large file. I can do that.

What I have problems with is that I want to check that the multiline string was appended. How would I do that with such a large file?

If the multiline string is appended I then want to remove it (or I want to do nothing if the file has something unexpected in it). How can I do this?

(I can do the above with a normal file, just not with a file greater than my PHP memory limit)

 

Have you tried using fseek to implement a tail() function? http://www.php.net/fseek