PHP: Function to Encrypt-Decrypt a string without a known key

The string itself has his own different key for every character

<?PHP
/*
Description : A function with a very simple but powerful xor method to encrypt
              and/or decrypt a string with an unknown key. Implicitly the key is
              defined by the string itself in a character by character way.
              There are 4 items to compose the unknown key for the character
              in the algorithm
              1.- The ascii code of every character of the string itself
              2.- The position in the string of the character to encrypt
              3.- The length of the string that include the character
              4.- Any special formula added by the programmer to the algorithm
                  to calculate the key to use
*/
function encrypt_decrypt($Str_Message) {
    $Len_Str_Message=strlen($Str_Message);
    $Str_Encrypted_Message="";
    FOR ($Position = 0;$Position<$Len_Str_Message;$Position++){
        // long code of the function to explain the algoritm
        //this function can be tailored by the programmer modifyng the formula
        //to calculate the key to use for every character in the string.
        $Key_To_Use = (($Len_Str_Message+$Position)+1); // (+5 or *3 or ^2)
        //after that we need a module division because can�t be greater than 255
        $Key_To_Use = (255+$Key_To_Use) % 255;
        $Byte_To_Be_Encrypted = substr($Str_Message, $Position, 1);
        $Ascii_Num_Byte_To_Encrypt = ord($Byte_To_Be_Encrypted);
        $Xored_Byte = $Ascii_Num_Byte_To_Encrypt ^ $Key_To_Use;  //xor operation
        $Encrypted_Byte = chr($Xored_Byte);
        $Str_Encrypted_Message .= $Encrypted_Byte;
        
        //short code of  the function once explained
        //$str_encrypted_message .= chr((ord(substr($str_message, $position, 1))) ^ ((255+(($len_str_message+$position)+1)) % 255));
    }
    RETURN $Str_Encrypted_Message;
} //end function

?>