Description:
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If you have zlib.output_compression = On in php.ini and echo some content then flush under apache 2 then you'll get some binary data due to the lack of headers.

I've had a look at the code and when you call flush() under Apache 2 it has

SG(headers_sent) = 1;
sapi_apache2.c line 265

During php_request_shutdown() sapi_send_headers is called to add appropriate gzip headers.

Within sapi_send_headers there is a check to see if headers have already been sent. SAPI.c line 701.

I've read previous bug reports on flush and Apache 2 in particular, bug #25701. Ilia mentions that Apache 2 and Apache 1 behave differently, on flush Apache 2 sends any pending headers but in this case the gzip headers haven't been added yet.

I can see two ways to handle this,
Disable zlib.output_compression if flush is called
or
Make flush do nothing if zlib.output_compression is enabled