Description:
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Many mb_ functions are not throwing type errors when the provided input does not match the type signature, on PHP 8.

Test script:
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<?php

function f(string $x) {}

var_dump((new ReflectionFunction('mb_strpos'))->getParameters()[0]->getType()->getName());
var_dump((new ReflectionFunction('f'))->getParameters()[0]->getType()->getName());

mb_strpos(null, 'a');
f(null);


Expected result:
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string(6) "string"
string(6) "string"

Fatal error: Uncaught TypeError: mb_strpos(): Argument #1 ($haystack) must be of type string


Actual result:
--------------
string(6) "string"
string(6) "string"

Fatal error: Uncaught TypeError: f(): Argument #1 ($x) must be of type string