Hi!
From what I understood reading the RFC, this looks like a new type,
"literal string", specifically a subtype of "string". Have you
considered instead of forcing library authors to use is_literal()
check everywhere on their input boundary and making sure no place
where input is passed, is forgotten, to let them type their whole
codebase, including internal, against the new literal-string type,
which would be enforced by PHP all the time? From my perspective as a
PHP developer, it would serve clearer intent than only checking in a
few places which presumably may take user input, i.e. you could
explicitly state "this function does only work with literal strings".
Regards,
someniatko