OneEditApart.php

<?php

/**
 * Make sure to enable zend.assertions=1 in your php.ini for assert to work.
 */
assert_options(ASSERT_ACTIVE, 1);

function OneEditApart(string $a, string $b): bool
{
    // the longer one will be on the left
    if(strlen($b) > strlen($a)) {
        $x = $a;
        $a = $b;
        $b = $x;
    }

    // if the length diff is more than 1 character, it's not one edit anymore
    if((strlen($a) - strlen($b)) > 1) {
        return false;
    }

    $ax = str_split($a);
    $bx = str_split($b);

    $i = 0;
    $j = 0;

    $hasChange = false;
    while($i < strlen($a)) {

        // string on the left is longer, so nothing more to compare to
        if(!isset($bx[$j])) {
            break;
        }

        // both letters are equal, lets try the next one
        if($ax[$i] === $bx[$j]) {
            ++$i;
            ++$j;
            continue;
        }

        // letters were not equal, if we had a change earlier, it's not one edit anymore
        if($hasChange) {
            return false;
        }

        // letters were not equal, therefore we note that here
        $hasChange = true;

        // if the $a is longer than $b, let's try to skip that letter and see if the next one will match
        if(strlen($a) > strlen($b)) {
            ++$i;
        } else {
            // nope, these are equal strings, lets try another one then
            ++$i;
            ++$j;
        }

    }

    return true;
}

// Tests
assert(OneEditApart('dog', 'cat') === false);
assert(OneEditApart('cat', 'cats') === true);
assert(OneEditApart('cat', 'cut') === true);
assert(OneEditApart('cat', 'cast') === true);
assert(OneEditApart('cat', 'at') === true);
assert(OneEditApart('cat', 'act') === false);
assert(OneEditApart('cat', 'acts') === false);