ob_flush() vs. flush(): When and Why Should You Use Both?

PHP Buffer Manipulation: Distinguishing ob_flush() from flush()

PHP provides two primary functions for manipulating output buffers: ob_flush() and flush(). Understanding their functional distinctions is crucial for effective buffer management.

ob_flush()

This function sends the contents of the application-initiated output buffer. Applications can initiate multiple nested ob_start() operations within a PHP script. ob_flush() passes the current buffered content to the next higher level.

flush()

In contrast, flush() flushes output buffers managed by PHP itself. This behavior depends on the backend used by PHP. Typically, FastCGI implements a socket buffer. To push contents to the web server, invoking flush() is essential.

Hierarchy of Buffering

Consider that the web server may also implement its own buffering scheme, influenced by configurations (e.g., mod_deflate or content filters). While these schemes are uncommon, they illustrate the multi-layered nature of buffering.

Recommendation

To ensure consistent and reliable output buffering, it is advisable to use both ob_flush() and flush() in conjunction. This practice guarantees that both application-initiated buffers and PHP-managed buffers are flushed effectively.

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