Can someone explain these two function lines to me.

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PHP

Hello, I don't quite understand what some of these lines do based on the previous code. Could someone explain to me ?

if ($c != " ") {
    $new_c = chr(((ord($c) - 97 + $clefChiffrage) % 26) + 97);

    return $alphabetFinal[ord($new_c) - 97];
}
return $c;

Context of this code : the goal of the code is to cypher a message based on the Caesar Code. It works by looking for the longest word in a string.

Based on this code, we get an encryption key. This is the number by which the translation to the right will be done. There is also an initial alphabet (abc....) and a final alphabet (azerty...).

What I have tried:

Here's the previous code :

$clefChiffrage = 0;
        $alphabetFinal = str_split("azertyuiopqsdfghjklmwxcvbn");

        function transformer($message) {
            $key_transform = array(    'Š'=>'S', 'š'=>'s', 'Ž'=>'Z', 'ž'=>'z', 'À'=>'A', 'Á'=>'A', 'Â'=>'A', 'Ã'=>'A', 'Ä'=>'A', 'Å'=>'A', 'Æ'=>'A', 'Ç'=>'C', 'È'=>'E', 'É'=>'E',
                            'Ê'=>'E', 'Ë'=>'E', 'Ì'=>'I', 'Í'=>'I', 'Î'=>'I', 'Ï'=>'I', 'Ñ'=>'N', 'Ò'=>'O', 'Ó'=>'O', 'Ô'=>'O', 'Õ'=>'O', 'Ö'=>'O', 'Ø'=>'O', 'Ù'=>'U',
                            'Ú'=>'U', 'Û'=>'U', 'Ü'=>'U', 'Ý'=>'Y', 'Þ'=>'B', 'ß'=>'Ss', 'à'=>'a', 'á'=>'a', 'â'=>'a', 'ã'=>'a', 'ä'=>'a', 'å'=>'a', 'æ'=>'a', 'ç'=>'c',
                            'è'=>'e', 'é'=>'e', 'ê'=>'e', 'ë'=>'e', 'ì'=>'i', 'í'=>'i', 'î'=>'i', 'ï'=>'i', 'ð'=>'o', 'ñ'=>'n', 'ò'=>'o', 'ó'=>'o', 'ô'=>'o', 'õ'=>'o',
                            'ö'=>'o', 'ø'=>'o', 'ù'=>'u', 'ú'=>'u', 'û'=>'u', 'ý'=>'y', 'þ'=>'b', 'ÿ'=>'y');
            $clean = strtr( $message, $key_transform);

            return strtolower($clean);
        }

        function cmp($str1, $str2) {
            $length1 = strlen($str1);
            $length2 = strlen($str2);

            if ($length1 == $length2)
                return 0;

            return ($length1 < $length2) ? -1 : 1;
        }

        function clefChiffrement($message) {
            $arr = explode(' ', $message);

            usort($arr, "cmp");

            return strlen(end($arr));
        }

        function _crypterMessage($c) {
            global $clefChiffrage;
            global $alphabetFinal;

            if ($c != " ") {
                $new_c = chr(((ord($c) - 97 + $clefChiffrage) % 26) + 97);

                return $alphabetFinal[ord($new_c) - 97]; 
            }
            return $c;
        }

        function crypter($message) {

            $arr = str_split($message);

            $arr_chiffre = array_map("_crypterMessage", $arr);

            return join("", $arr_chiffre);
        }