Update test-optimization.php

[nextend]

Amends #1317

See WordPress/gutenberg#62770

WordPress/gutenberg#62770 (comment):

@westonruter I think the missing piece is to call ob_end_flush(); before calling ob_get_clean();.

On line 91, a new output buffer is created. This output buffer is necessary as it allows you to access the output in the same way the PHP runtime does when outputting the default output buffer.

In the original code, the following happened:

• System [1] output buffer <- This will be sent to the browser.

  • Line 91 -> [2] output buffer

    • od_buffer_output -> [3] output buffer
    • This part is tricky: Calling ob_get_contents() at this point does not trigger the ob callback, as there might be more text to add to the buffer after calling ob_get_contents. Example: https://gist.github.com/nextend/ac5c90de0e2d628fba02355819998a34
    • If you check the source code of PHP, ob_get_clean has two parts: first, it calls ob_get_contents, and then it discards the output buffer. This is where a common misunderstanding occurs. In the first example, I showed why ob_get_contents does not trigger the ob callback, and since it is used inside ob_get_clean, there will be no ob callback call. Reference: https://github.com/php/php-src/blob/48916cec3383e5ac83bd9486d5445f847a36b922/main/output.c#L1408

In the new code:

• System [1] output buffer <- This will be sent to the browser.

  • Line 91 -> [2] output buffer

    • od_buffer_output -> [3] output buffer
    • ob_end_flush -> Output buffer [3] is closed -> ob callback gets called -> Result of the callback is flushed into [2] buffer

  • Now we can access the real output with ob_get_clean, which gives us the content of the [2] buffer (and then discards the output).

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