Return type of `substr` with constants depends on runtime PHP version instead of analyzed PHP version

[reima]

Bug report

Description

In PHP <8.0 substr returns false in some cases. In PHP >=8.0 substr returns "" instead in these cases. See https://3v4l.org/6aUKU

When analyzing substr calls with constant arguments, PHPStan uses the substr behavior of the runtime PHP version instead of the PHP version that is being analyzed.

This means that even with a fixed PHP version to analyze (via phpVersion in the config, or composer.json), PHPStan might deduce different types for substr when running on PHP <8.0 vs. running on PHP >=8.0.

Repro

It's hard to show the bug on https://phpstan.org/try, as there is not way to change the runtime PHP version or the analyzed PHP version (at least to my knowledge).

This is the best I've come up with to illustrate the expected behavior: https://phpstan.org/r/b7f81641-ee04-44e8-87f2-b02d11e29c06

Hints

I've almost implemented a fix myself, but couldn't figure out how to test this properly. But I'll share my findings here.

In order to fix this three cases must be considered:

• The substr behavior of the runtime PHP version and the analyzed PHP version match: Use the runtime substr.
• The runtime PHP version returns false, and the analyzed PHP version returns "": Use the runtime substr and substitute false with "".
• The runtime PHP version returns "", and the analyzed PHP version returns false: Find out if this is one of the cases where false should be returned. Otherwise, use the runtime substr.

For the last case the substr implementation of PHP 7.4.33 can be consulted. Concentrating on the cases where false is returned, the logic can be implemented as follows:

function substrBeforePhp8(string $string, int $offset, ?int $length = null): false|string
{
	$strlen = strlen($string);

	if ($offset > $strlen) {
		return false;
	}

	if ($length !== null && $length < 0) {
		if ($offset < 0 && -$length > $strlen) {
			return false;
		}
		if ($offset >= 0 && -$length > $strlen - $offset) {
			return false;
		}
	}

	return substr($string, $offset, $length);
}

Code snippet that reproduces the problem

https://phpstan.org/r/b7f81641-ee04-44e8-87f2-b02d11e29c06

Expected output

The result type of substr should be determined based on the analyzed PHP version.

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[VincentLanglet]

Thanks for the report, I tried phpstan/phpstan-src#4061