Fix #13963 with unit tests.

[niconoe-]

Fixes phpstan/phpstan#13963

[VincentLanglet]

Hi, I think you're not handling correctly the flag for non-string value

I recommend you to add test for

filter_var(true, options:FILTER_FLAG_EMPTY_STRING_NULL); // non-empty-string
filter_var(false, options:FILTER_FLAG_EMPTY_STRING_NULL); // null
filter_var($bool, options:FILTER_FLAG_EMPTY_STRING_NULL); // null
filter_var(0.0, options:FILTER_FLAG_EMPTY_STRING_NULL); // non-empty-string
filter_var(0, options:FILTER_FLAG_EMPTY_STRING_NULL); // non-empty-string
filter_var(null, options:FILTER_FLAG_EMPTY_STRING_NULL); // null
filter_var($nullable, options:FILTER_FLAG_EMPTY_STRING_NULL); // null|non-empty-string

You certainly need to do the check you did but after a toString call.

[niconoe-]

Thanks for your feedback! I added some cases and tried to be as precise as I could on type expectations. I feel like it's starting to get too complex when going deeper on union types. I'm not feeling fluent enough with PHPStan's API but I did my best.

Again, tell me if I got something wrong here 😉

[VincentLanglet]

I feel like it could be simpler to look for something like

if ($filterValue === $this->getConstant('FILTER_DEFAULT')) {
             $scalarOrNull = new UnionType([
				new StringType(),
				new FloatType(),
				new BooleanType(),
				new IntegerType(),
				new NullType(),
			]);
			if ($scalarOrNull->isSuperTypeOf($in)->yes()) {
				$canBeSanitized = $this->canStringBeSanitized($filterValue, $flagsType);
				if ($canBeSanitized->no()) {
					$stringType = $in->toString();
				} else {
					$stringType = $in->isString()->no()
						? $in->toString()
						: TypeCombinator::union(TypeCombinator::remove($in, new StringType()), new StringType());
				}

				return $this->handleEmptyStringNullFlag($stringType, $flagsType);
			}
		}

with

private function handleEmptyStringNullFlag(Type $in, ?Type $flagsType): Type
	{
		$hasFlag = $this->hasFlag('FILTER_FLAG_EMPTY_STRING_NULL', $flagsType);
		if ($hasFlag->no()) {
			return $in;
		}

		$hasEmptyString = !$in->isSuperTypeOf(new ConstantStringType(''))->no();
		if ($hasFlag->maybe()) {
			return $hasEmptyString ? TypeCombinator::addNull($in) : $in;
		}

		return $hasEmptyString ? TypeCombinator::remove(TypeCombinator::addNull($in), new ConstantStringType('')) : $in;
	}

Some existing tests will fail, but I think it's an improvement.
There will be only

		$return = filter_var($nullable_string, options: FILTER_FLAG_EMPTY_STRING_NULL);
		assertType('non-empty-string|null', $return);

to fix. This is something related to (I think)

if ($exactType === null || $hasOptions->maybe() || (!$inputType->equals($type) && $inputType->isSuperTypeOf($type)->yes())) {
			if (!$defaultType->isSuperTypeOf($type)->yes()) {
				$type = TypeCombinator::union($type, $defaultType);
			}
		}

[VincentLanglet]

I think you could try something like https://github.com/phpstan/phpstan-src/compare/2.1.x...VincentLanglet:phpstan-src:fix/filter_var?expand=1 with all your existing tests @niconoe-