What about boolval?

I'm not aware of any restrictions on boolval. As far as I know it really accepts mixed. Am I missing something?

Also I think I wiill remove everything but strval. Only strval needs special handling due to Stringable. Everything else should be fixable by changing the function signature in functionMap.

Is there a better way, to check if this is an instance of any object?

Is it ever useful to allow the array and resource here?

I'm also a bit confused - doubleval does not accept object here which should already be picked up by an existing rule (CallToFunctionParametersRule) but apparently isn't. Any idea why?

Is it ever useful to allow the array and resource here?

Probably not. I was attempting to keep (float) and floatval() in line and used the actual PHP behavior as a guideline along with the fact that casting to int is possible.

I'm also a bit confused - doubleval does not accept object here which should already be picked up by an existing rule (CallToFunctionParametersRule) but apparently isn't. Any idea why?

I have to check again, but if I remember correctly, it does pick up regular objects, but has a special rule to allow objects with a __toString-method. Which in general is the correct behavior. intval and floatval are the special case, in that they accept string but not Stringable:

php > $stringable = new class() {public function __toString(): string {return '';}};
php > echo strlen($stringable);
0
php > echo intval($stringable);
PHP Notice:  Object of class class@anonymous could not be converted to int in php shell code on line 1

I have added a commit to forbid casting resource to float. I'm looking into doing the same with array but that causes a regression for phpstan/phpstan#5162.

So I looked into phpstan/phpstan#5162. The problem is, that the fix for that issue was to explicitly allow casting array to float. If we forbid it here again, this will obviously fail again. In order to resolve this, the underlying issue would have to be resolved:

/** @var array<string,string|array<string>> $result */
$result = $this->Get();
if ( array_key_exists('a',$result) && ! is_numeric( $result['a'] ) )
{
	exit(1);
}
assertType('numeric-string', $result['a']); //this currently reports 'array<string>|string'

/** @var array<string,string|array<string>> $result2 */
$result2 = $this->Get();
if ( ! is_numeric( $result['a'] ) )
{
	exit(1);
}
assertType('numeric-string', $result['a']); //this works

As far as I can tell in the first case the resolved type of $result['a'] is not updated, because it might not even exist. Obviously the type of $result['a'] could also be ERROR if the offset doesn't exist.
However I have no idea how to fix this.