• Is output buffering on? If so, you have to use the ob_ functions.
• Do you see the Transfer-Encoding: chunked response header? Chunked encoding is typically required for flushing to work.
• Is the response gzipped? If so, and you’re running a version of Apache earlier than 2.2.8, you have to add padding to your page.
• Are you behind a proxy or using antivirus software? These might buffer the chunks before sending them through to the browser.
• Are any of the resources referenced in the flushed chunk being blocked because they’re fetched from the same domain as the HTML document?
• Are you testing only in Safari or Chrome? The flushed HTML must be more than 2 KB to see the benefits in these browsers.
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