Can I use ob_clean() without ob_start() before?

I am not used to ob_* functions and I began by reading the documentation.
I would like to use ob_clean() in a method but the documentation says:

The output buffer must be started by ob_start() with PHP_OUTPUT_HANDLER_CLEANABLE flag. Otherwise ob_clean() will not work.

However I tried (just to see how it "wouldn't work"):

echo 'a';
ob_clean();
echo 'b';

Which outputs b. Everything seems working fine here, but following the documentation, it shouldn't, should it ?

• Is ob_start() mandatory for ob_* functions ?
• Is there a variable (in the php.ini for example) which default value allows to ommit ob_start() ?
• How should I understand "ob_clean() will not work" ? What should I expect ?

The short answer is that yes, you have to turn on output buffering before you can use ob_clean. However, it is possible that you have output buffering enabled for all files through the output_buffering ini parameter (see the manual). The way to check for this is through ob_get_level():

echo ob_get_level();

If the result is non-zero, output buffering is enabled.

If you do use ob_clean() without buffering enabled, you should see an error similar to this:

Notice: ob_clean() [ref.outcontrol]: failed to delete buffer. No buffer to delete. in [file-path]