Distance between strings in PHP

DistanceBetweenStringSets.php

<?php
/**
 * Created by PhpStorm.
 * User: adriano
 * Date: 11/11/19
 * Time: 21:51
 */

function programmerStrings( $s ) {
	$possibleLetters = getPossibleLetters();
	$stringArr       = str_split( $s );
	$maxSpace        = 0;
	$currentLen      = 0;
	$wordLen         = 2;
	$end             = null;
	$start           = null;

	foreach ( $stringArr as $key => $letter ) {
		if ( isset( $possibleLetters[ $letter ] ) ) {
			if ( $possibleLetters[ $letter ] > 0 ) {
				if ( $start === null ) {
					$start = $key;
				}

				$currentLen ++;
				$possibleLetters[ $letter ] --;
			} else {
				$start = $key;
			}
		}

		if ( $currentLen === $wordLen ) {
			if ( $start && $end ) {
				$space = $start - $end - 1;

				if ( ! $maxSpace || $space > $maxSpace ) {
					$maxSpace = $space;
				}
			}

			$end             = $key;
			$start           = null;
			$possibleLetters = getPossibleLetters();
			$currentLen      = 0;
		}
	}

	return $maxSpace;
}

function getPossibleLetters() {
	return [
		'h' => 1,
		'i' => 1,
	];
}

assert( 0 === programmerStrings( 'hihi' ) );
assert( 0 === programmerStrings( 'hi' ) );
assert( 2 === programmerStrings( 'hiiiihi' ) );
assert( 5 === programmerStrings( 'hiqqqqqhi' ) );
assert( 1 === programmerStrings( 'hiiih' ) );
assert( 8 === programmerStrings( 'hiiiihiiiiiiiiih' ) );