Is the PHP opening tag mandatory in byte count?

Question

After answering the first time on PPCG on "Source code ecological footprint" I had a little discussion about it. I found it's OK if your program throws notices or warnings. But I couldn't find anything about the opening PHP tag.

My question is: Do I need to include and count it? If yes, is the short tag <? instead of <?php allowed, as it must be enabled via --enable-short-tags or php.ini?

In any case, could somebody please add an explanation in their answers on why it is needed or not? I think this might be useful for other users as well as currently all different versions can be found here in PHP answers.

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Interesting read as well: Running PHP with -r instead of code tags. If a program is run from the command line using -r option it doesn't need the opening tag, like:

php -r "echo 'ok';"

_{I hope this is not a duplicate, found nothing related when searching for the tag and word "PHP".}

Answer 1

As mentioned here, running programs via commands such as php -r "code here" is acceptable without penalty.

Since <? and <?php are both unneeded (and in fact not allowed) when using php -r, it follows that neither tags are required.

Answer 2

Regarding the use of the <? short tag: since it's on by default, I think the use of <? instead of <?php is okay for code golf.