assuming that you are committing only this file, the reason could be that you realy have an older version ( -r 1951 filename.php) in your working copy. Commit will need you to update your file to latest revision.
If you use svn update on this file you will get a conflict message something like this:
You need to resolve this and only after that you can proceed.
Conflict discovered in filename.php'.
Select: (p) postpone, (df) diff-full, (e) edit,
(mc) mine-conflict, (tc) theirs-conflict,
(s) show all options: