When we say that a function's result is sent to output buffer, it means that the result is not visible until echoed. And for functions that output directly, we use ob_start to direct the result to the output buffer (before it hits the browser as plain html) so that we may manipulate it, and then if desired, echo it. The result of the function readfile() is directly visible, which is the contents of a certain text file, for example. then my question is:
Why it is mentioned in php documentation that readfile() sends the contents to the output buffer ?! (while in fact it is displayed directly).
Am I missing something?
When we say that a function's result is sent to output buffer, it means that the result is not visible until echoed. No it doesn't mean that!
Because it is sent to the output buffer, which (unless you've explicitly executed an ob_start() to buffer the output before sending it to the browser) is sent directly to the browser (or whatever). All output is sent to the output buffer, but unless you've set the buffer to hold output before sending it, it gets sent.
EDIT
Of course, a webserver may be configured to cache output until a certain volume of data has been sent, but that isn't the same as PHP output buffering