An insitu (no copy) string comparison comparable to substr_compare()

instr.php

<?php

function instr(string $string, string $substr, int $offset = 0, int $length = null): bool
{
	for ($pos = $offset, $substr_pos = 0; $pos !== $length; $pos++, $substr_pos++)
	{
		if (isset($substr[$substr_pos]))
		{
			if (isset($string[$pos]))
			{
				if ($string[$pos] !== $substr[$substr_pos])
					return false;
			}
			else
				return false;
		}
		else
		{
			if ($length === null || $length + 1 === $substr_pos)
				return true;

			if (isset($string[$pos]))
				return false;
		}
	}

	return true;
}

function test($haystack, $needle) {
	$repetitions = 0;
	$start = microtime(true);
	do {
		strpos($haystack, $needle);
		$repetitions += 1;
	} while (microtime(true) - $start < 5);

	$rate = round($repetitions / (microtime(true) - $start));
	$rate = str_pad($rate, 8, ' ', STR_PAD_LEFT);

	echo "$rate / second\n";

	$repetitions = 0;
	$start = microtime(true);
	do {
		instr($haystack, $needle);
		$repetitions += 1;
	} while (microtime(true) - $start < 5);

	$rate = round($repetitions / (microtime(true) - $start));
	$rate = str_pad($rate, 8, ' ', STR_PAD_LEFT);

	echo "$rate / second\n\n";
}

// Worst case
test(str_repeat('A', 1e+7) . 'BB', 'BB');
test(str_repeat('aA', 1e+6) . 'aB', 'aB');
test(str_repeat('A', 1e+1) . 'BB', 'BB');
test(str_repeat('aA', 1e+1) . 'aB', 'aB');
// Best case
test(str_repeat('a', 1e+7), 'aa');
test(str_repeat('aa', 1e+6), 'aa');
test(str_repeat('a', 1e+1), 'aa');
test(str_repeat('aA', 1e+1), 'aa');


$string = 'abcdefghijklmnopqrstuvwxyz';
$substr = 'def';
$offset = 3;
$length = 0;

var_dump(instr($string, $substr, $offset, $length));

php -v
PHP 8.1.1 (cli) (built: Dec 15 2021 10:31:43) (ZTS Visual C++ 2019 x64)

11th Gen Intel(R) Core(TM) i7-11700F @ 2.50GHz

    3847 / second
 9553267 / second

     297 / second
 7184814 / second

14038493 / second
 9540673 / second

10923208 / second
 7027445 / second

14676242 / second
 6136964 / second

14933345 / second
 6169539 / second

14700096 / second
 6136005 / second

11542197 / second
 7023714 / second