I have encountered a similar problem described here (and in other places) -
where as on an ajax callback I get a xmlhttp.responseText that seems ok (when I alert it - it shows the right text) - but when using an 'if' statement to compare it to the string - it returns false.
(I am also the one who wrote the server-side code returning that string) - after much studying the string - I've discovered that the string had an "invisible character" as its first character. A character that was not shown. If I copied it to Notepad - then deleted the first character - it won't delete until pressing Delete again.
I did a charCodeAt(0) for the returned string in xmlhttp.responseText. And it returned 65279.
Googling it reveals that it is some sort of a UTF-8 control character that is supposed to set "big-endian" or "small-endian" encoding.
So, now I know the cause of the problem - but... why does that character is being echoed?
In the source php I simply use
echo 'the string'...
To conclude, and specify the solution:
Windows Notepad adds the BOM character (the 3 bytes: EF BB BF) to files saved with utf-8 encoding.
PHP doesn't seem to be bothered by it - unless you include one php file into another - then things get messy and strings gets displayed with character(65279) prepended to them.
You can edit the file with another text editor such as Notepad++ and use the encoding
"Encode in UTF-8 without BOM",
and this seems to fix the problem.
Also, you can save the other php file with ANSI encoding in notepad - and this also seem to work (that is, in case you actually don't use any extended characters in the file, I guess...)