Description:
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The manual for ob_start() provides the following description for the chunk_size parameter:

"If the optional parameter chunk_size is passed, the callback function is called on every first newline after chunk_size bytes of output. Default value 0 means that the function is called only in the end, other special value 1 sets chunk_size to 4096."

I believe There are two problems with that statement.

1. Passing the chunk_size does not just cause the callback function to be called. It causes a flush of the buffer, which will in turn trigger the callback function, if one is available.
This is important, since the documentation explains how it is possible to set a chunk_size and erase flag without a callback function by providing null as the first argument.

2. The flush does not occur "on every first newline after chunk_size bytes". It occurs after any append to the buffer which has caused the buffer length to equal or exceed the chunk_size. Newlines are not treated differently from other characters.
Evidence for this is in the code in output.c in php_ob_append():

  if (OG(active_ob_buffer).chunk_size
    && OG(active_ob_buffer).text_length >= OG(active_ob_buffer).chunk_size) {
      php_end_ob_buffer(1, 1 TSRMLS_CC);
      return;
  }
	

So, I think the statement should read:

If the optional parameter chunk_size is passed, the buffer will be flushed after any output call which causes the buffer's length to equal or exceed chunk_size.
There are two special values:
  0 (default value): the buffer is flushed only when explicitly called for (for example with ob_flush()) or at the end of the request.
  1: equivalent to setting chunk_size to 4096.
Note that flushing the buffer will trigger the callback function, if it is available.

Reproduce code:
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Here is a test demonstrating that a buffer is flushed after an output call causes its length to equal or exceed chunk_size, and that this is not dependent on the occurrence of new lines:
http://pastebin.com/f46cbdf76

Expected result:
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N/A

Actual result:
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N/A