Returns TRUE if the two input strings form anagrams of each other otherwise FALSE

anagrams.php

<?php

$string1 = strtolower(preg_replace('/\s+/', '', "AstroNomers")); // Prepare string
$string2 = strtolower(preg_replace('/\s+/', '', "no more stars")); // Prepare string

$bool = isAnagram ( $string1, $string2 );

/**
 * Returns TRUE if the two input strings form anagrams of each other otherwise FALSE.
 * Also return FALSE in case of invalid inputs.
 *
 * @param $string1
 * @param $string2
 *
 * @return bool
 */
function isAnagram($string1, $string2)
{
    if (!is_string($string1) || !is_string($string2)) {
        return false;
    }

    $str = str_split($string2);
    $key = array_search($string1[0], $str);

    if ($key !== false) {
        unset($str[$key]);
    }

    $string1 = substr($string1, 1);
    $string2 = !empty($str) ? implode("", $str) : false;

    if ($string1 != false && $string2 != false) {
        return isAnagram($string1, $string2);
    }

    return $string1 == false  && $string2 == false;
}