a compatibility issue:
when pointers to a file (ex. $file = fopen("file", "a"); ) are used in function
fputs using quotes, like so:

fputs("$file", "Text blah blah");
it does not do the function and tells you it is an invalid file pointer.

this would work with no problems in the 3.X releases.  the simple fix is to remove the quotes, and then it works fine.  I do not know if this problem exists beyond fputs()..