Description:
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Currently PHP doesn't check the ETag header at all, nor does it send it out. I propose to add an ETag functionality that works like this:

1. in the output zlib handler check whether an ETag (If-None-Matched) was set, and store it in the ZLIB globals.
2. when we're compressing the output we do an MD5 (or any other kind of hashing function) over the compressed output.

3. If the hash is the same as the If-None-Matched header value that was stored in step 1: just send a 304 Not Modified, if it's not the same send the normal output but at an ETag header containing the hash value.