[PHP] filemtime doesn't always copy correctly

[David T. Kotchen]

I am  backing up a file (the path & filename of which are stored in  $original; 
the path & filename of the copy are stored in $bkup). I  would also like to 
copy the file modification time of $original, so,  after copying the file 
itself, I use:
  
  touch($bkup, filemtime($original));
  
However, after I use the above "touch" function,  filemtime($original) and 
filemtime($bkup) are often slightly different.  For example, I might have:
  
  echo filemtime($original); // gives 1126191599
  
  echo filemtime($bkup); // gives 1126191600
  
  Why the small difference?
  
  (I'd like these two file modification times to be exactly the same  because I 
want to compare file modification times when I do incremental  backups, and 
copy a file only if the file modification times are  different.)
  
  By the way, if it helps, I am using Windows XP.
  
  Thanks!