Description:
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The documentation of ob_start says that if chunk size is specified, then the handler is called on the first newline in the output.

This did not make sense since what if the output was binary and had no new lines chunking would not work at all, I tested and found that the handler instead "snaps" to the first boundary after an echo is performed.

I.e. each echo operation is atomic, so if I had a chunk size 10 bytes, and I echo '123456', '123456', '123456', '123456'; then the handler will be called with 2 times, with 12 bytes each time, although there are no newlines in the output.