Question

The problem I’m facing is quite common I suppose but I didn’t see a single thread in internet.

If I use isset($_POST),

1. Will it always return true?
2. Does the response depend on the version of PHP I use?
3. Is $_POST is a variable? (it’s a super global ‘variable’ after all). Because in php.net documentation, it is mentioned

isset() only works with variables as passing anything else will result in a parse error.

Answer 1

Will it always return true?

Yes, even if the page was opened using GET method or nothing was POSTed.

Does the response depend on the version of PHP I use?

No it does not (not sure about very old versions of PHP).

Is $_POST is a variable?

Yes

isset() only works with variables as passing anything else will result in a parse error.

This is explicitly mentioned in the manual so that people do not try to do cheaky stuff. These won’t work for example:

function getVarName() { return '_POST'; }
isset(getVarName());
isset('$_POST');

Now, why would you want to check if $_POST is set. Perhaps want to check if a certain variable (e.g. email) was posted, in that case you need to check:

isset($_POST["email"])