FTP Corrupts PHP by Inserting Characters

Question

I have been using Dreamweaver css.5 for many years and suddenly it has started corrupting files. The Site is one I have been developing for a very long time and the settings of that haven't changed recently. The problem just appeared without any cause that I can think of.
If I upload the file with DW a small chunk of PHP that is at the beginning of the file appears about 500 or so lines down.
So, this is how my file starts:
<?php
// categories defined in AdSubCats
define('BUY_CATEGORY', 1); 
On line 508 of the uploaded file this appears (but the exact same file uploaded with Filezilla doesn't)
if($this->m<?php
// categories defined in AdSubCats
deerVisible()) {
It happens with every file that I upload but not at predictable lines or byte positions.
I have tried everything I can think of including:
Downloading and reinstalling DW css.5
Getting all updates
Changing the LF types suggested above.
Deleting the configuration files, cache and _notes
Restarting my PC + Router
With Filezilla the files upload and work fine, but with DW they get corrupted and the problem just suddenly started yesterday.
Any ideas please?

Answer 1

This really is so weird!! I have tried absolutely everything I can think of to fix this.
Recreating the Site umteen times, changing the settings - passive mode etc
Reinstalling Dreamweaver
Recreating the Site
Restarting my PC
Restarting and Resetting the Router
Still it happens - to all files that I upload with Dreamweaver.
Not at predictable byte positions - eg. 16326 or 16385 but always about line 500.
It was working fine till 2 days ago - and has been working fine for YEARS!!
Here's another example, these are the lines at the beginning of the PHP file:
<?php
if($_SERVER['HTTP_HOST']==
and they get copied to line 484 into the middle of some HTML:
<!-- end #mainContent --></div><?php
if($_SERVER['HTTP_HOST']==
  <!-- end #footer --></div>
Upload the exact same PHP file with Filezilla and it's perfectly Hunky Dory.
It's as though the DW FTP client is getting something confused in the blocks that it is sending - but why?
Has anyone any ideas, please?