[VE][400] Error Message Feedback

Validating Error [400]: "processing instruction does not start with name"

The mail search link provided when checking on this error does not produce any results:
http://www.w3.org/Search/Mail/Public/search?type-index=www-validator&index-type=t&keywords=%5BVE%5D%5B400%5D&search=Search

What caused the warning (that should be an error)
In mass-changing some PHP files to have an "include" statement, I accidentally changed some HTML files to have the include statement as well. Of course, these do not display properly.  The files passes validation but gives the warning.  At first, I could not see what was wrong since I was looking at the file contents as if it were a PHP file.  I am not sure what else can cause this warning but since the file will not display correctly, to me it should be an error.  If it could be determined that a PHP code sequence has been inserted to an HTML file, then the error message should explain this possibility.

Since the original file has been replaced, I do not have a reference to point you to.  Any PHP file with an include statement, renamed to an HTML extension should produce the same error.

I understand that some server side includes could be implemented to work with files having the HTML extension, however, I am not aware of any servers that have this feature.  If they exist, surely they will not use the same syntax as the PHP include statement.

Best of luck.

Thanking you in advance;

GCS