Today, I have looked into HTML code of facebook.com, and found something like this:
<input type="hidden" value="€,´,€,´,水,Д,Є" name="charset_test"/>
It's repeated two times inside the <form>...</form>.
Any idea what this code might be useful for - some kind of server-side client charset detection? As far as I know, browser charset is being transmitted in HTTP request anyway (an "Accept-Charset" header).
I guess they are matching this in the receiving script to make sure the client sent the request properly encoded as UTF-8 and maybe even, because they know what characters to expect, to detect the actual encoding on the fly.
If I remember correctly - I had to deal with it once - there have been problems with form encoding in IE6 in some situations.
€,´,€,´,水,Д,Є
I guess some browser send € same as € and ´ same as ´,
So they can check like charset_test[0] == charset_test[2] and charset_test[1] == charset_test[3]
For others other characters, I have no clue. 水 probably test for CJK.
As Pekka says, this is to be able to detect the request charset. The HTTP protocol doesn't provide a way to specify the charset of a request. Because of this, one has to rely on conventions outside of the protocol. Generally browsers are predictable, but this trick is the only way to be 100% sure.
See also: http://www.phpwact.org/php/i18n/charsets
Any idea what this code might be useful for - some kind of server-side client charset detection?
Apparently so.
The Euro sign is useful for charset detection because there are so many ways of encoding it:
As far as I know, browser charset is being transmitted in HTTP request anyway (an "Accept-Charset" header).
It's supposed to transmitted in the HTTP Content-Type header, but that doesn't mean that user agents actually get it right.