Above usage of filter_var is wrong, there is no need to cast $thing to int, filter_var already returns the filtered int or false on failure. The correct usage is therefore

    /**
     * Cast to int, fail if not a valid integer
     * @throws \UnexpectedValueException
     * @param mixed $thing
     * @return int
     */
    function as_int($thing): int                                                                                             
    {   
       $int = filter_var($thing, \FILTER_VALIDATE_INT);
       if ($int === false) {
         throw new \UnexpectedValueException("Value is not an integer.");
       }
       return $int;
    }

Thus the critique against PHP that is lacks a proper integer parsing function is simply not true, filter_var has been part of PHP since 5.2 released in 2006, over 16 years ago. filter_var works as advertised, because I use it almost every time i write PHP and that is to mostly to parse integers but sometimes also for other types. And I don't even wrap it with my own as_int function, using it inline with an simple if-check afterwards is actually cleaner. And if you prefer that filter_var returns null on failure instead of false you can simply add FILTER_NULL_ON_FAILURE as the third argument to filter_var.

And this is the recurring thing with PHP critique, vast majority of the time it just in imagination because nobody bothers to look it up.

https://www.php.net/manual/en/function.filter-var.php