I'm not sure if this could be easily built into PHP, but per-directory options are supported by PHP (when on apache, via a .htaccess file). If you could turn on or off strong typing per directory (or, alternatively, per file much like JS's "use strict"), there wouldn't be much of a practical problem.
It could even be a function modifier.
Unlike static typing or const correctness or immutability, strong typing is a very local feature that doesn't "bubble" across your codebase once you start using it.
Allowing .htaccess comes with a performance hit (Apache has to scan the directory and all of its parents for a .htaccess file on every pageview) and should generally not be used unless strictly necessary.
It could even be a function modifier.
Unlike static typing or const correctness or immutability, strong typing is a very local feature that doesn't "bubble" across your codebase once you start using it.