Anagrams

<?php
function anagram($check_anagram_string) {

    $check_string_length = strlen($check_anagram_string);

    if ($check_string_length % 2 == 1) {
        // is odd string, can not compare
        return -1;
    }

    // convert to lower case
    $check_anagram_string = strtolower($check_anagram_string);

    // split string to different variable
    $sub_string_index = (int) floor($check_string_length / 2);
    $compare_string_1 = substr($check_anagram_string, 0, $sub_string_index);
    $compare_string_2 = substr($check_anagram_string, -$sub_string_index);

    $check_letter_mapping = [];

    for ($check_sting_1_index = 0; $check_sting_1_index < $sub_string_index; $check_sting_1_index++) {
        $letter = $compare_string_1[$check_sting_1_index];
        if (!isset($check_letter_mapping[$letter])) {
            // the letter never exist
            $check_letter_mapping[$letter] = 1;
            continue;
        }

        // add letter show times
        $check_letter_mapping[$letter]++;
    }


    for ($check_sting_2_index = 0; $check_sting_2_index < $sub_string_index; $check_sting_2_index++) {
        $letter = $compare_string_2[$check_sting_2_index];
        if (!isset($check_letter_mapping[$letter])) {
            // the letter never exist
            $check_letter_mapping[$letter] = -1;
            continue;
        }

        // letter already exist in string 1, delete compare times
        $check_letter_mapping[$letter]--;
    }

    $change_to_anagram_change_times = 0;

    foreach ($check_letter_mapping as $letter => $letter_compare_result) {
        if ($letter_compare_result > 0) {
            $change_to_anagram_change_times+=$letter_compare_result;
        }
    }

    return $change_to_anagram_change_times;
}