Anagram test (PHP, multi-byte strings)

anagram.php

<?php 

function isAnagram($string1, $string2) {
	if (!is_string($string1) || !is_string($string2)) { 
		return false;
	}
	
	$map1 = [];
	$map2 = [];
	
	for ($i = 0; $i < mb_strlen($string1); $i++) {
		$char = mb_strtolower(mb_substr($string1, $i, 1));
		if (!ctype_space($char)) {
			if (isset($map1[$char])) {
				$map1[$char]++;
			} else {
				$map1[$char] = 1;
			}
		}
	}
	
	for ($i = 0; $i < mb_strlen($string2); $i++) {
		$char = mb_strtolower(mb_substr($string2, $i, 1));
		if (!ctype_space($char)) {
			if (isset($map2[$char])) {
				$map2[$char]++;
			} else {
				$map2[$char] = 1;
			}
		}
	}
	
	return $map1 == $map2;
}

$anagramTests = [
	['a  ', '  A', true],
	['a', 'b', false],
	['HA HA N N', 'hannah', true],
	['', '', true],
	[2, 2, false],
	['ABCD', '一只快过速的棕色狐狸跳过懒狗', false],
	['速的棕色', '色  棕  的  速', true]
];

foreach ($anagramTests as $test) {
	$testResult = isAnagram($test[0], $test[1]);
	$assertFailMessage = "{$test[0]} and {$test[1]} are " . ($testResult ? 'anagrams,' : 'NOT anagrams,') . ' expected the opposite.';
	assert($testResult === $test[2], $assertFailMessage);
}

?>