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

instr.php

<?php


function instr($string, $substr, $offset = 0, $length = -1)
{
	$pos = (int) $offset;
	$substr_pos = 0;

	for (; $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 === -1 || $length + 1 === $substr_pos)
			{
				return true;
			}

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

	return true;
}

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

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