Description:
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Mixed (type) it's not a mixed in other language (java, c, etc);
It's a unique type, because  string, array, any class and other not = mixed or \mixed;


Test script:
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public function changeValue(string $name, \mixed $data): ClassName {}

$object = new ClassName();
$object->changeValue('name', 'email')->getResult();


Expected result:
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FatalThrowableError in ClassName.php line 54:
Type error: Argument 2 passed to App\Http\ResponseDataService\ClassName::changeValue() must be an instance of mixed, string given