Is it true that string literals in PHP can only be encoded in an encoding which is a compatible superset of ASCII, such as UTF-8 or ISO-8859-1?

I come across following text from the Details of the String Type page from PHP Manual :

Given that PHP does not dictate a specific encoding for strings, one might wonder how string literals are encoded. String will be encoded in whatever fashion it is encoded in the script file. Thus, if the script is written in ISO-8859-1, the string will be encoded in ISO-8859-1 and so on. However, this does not apply if Zend Multibyte is enabled; in that case, the script may be written in an arbitrary encoding (which is explicity declared or is detected) and then converted to a certain internal encoding, which is then the encoding that will be used for the string literals. Note that there are some constraints on the encoding of the script (or on the internal encoding, should Zend Multibyte be enabled) – this almost always means that this encoding should be a compatible superset of ASCII, such as UTF-8 or ISO-8859-1.

So my doubt is, is it true that string literals in PHP can only be encoded in an encoding which is a compatible superset of ASCII, such as UTF-8 or ISO-8859-1 and not in an encoding which is not a compatible superset of ASCII?

Is it possible to encode string literals in PHP in some non-ASCII compatible encoding like UTF-16, UTF-32 or some other such non-ASCII compatible encoding? If yes then will the strings literals encoded in such one of the non-ASCII compatible encoding work with mb_string_* functions? If no, what's the reason behind not able to encode a string literal in non-ASCII compatible encoding?(This is the actual doubt I have indeed)

Suppose, Zend Multibyte is enabled and I've set the internal encoding to a compatible superset of ASCII, such as UTF-8 or ISO-8859-1 or some other non-ASCII compatible encoding. Now, can I declare the encoding which is not a compatible superset of ASCII, such as UTF-16 or UTF-32 in the script file?

If yes, then in this case what encoding the string literals would get encoded in? If no, what's the reason?

Also, explain me how does this encoding thing work for string literals if Zend Multibyte is enabled?

How to enable the Zend Multibyte? What's the main intention behind turning it On? When it is required to turn it On?

It would be better if you could clear my doubts accompanied by suitable examples.

Thank You.

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