PHP Function to EncryptDecrypt a string without a known key

<?php

//The string itself has his own different key for every character
FUNCTION ENCRYPT_DECRYPT($Str_Message) {
//Function : encrypt/decrypt a string message v.1.0  without a known key
    $Len_Str_Message=STRLEN($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

//sample use of the function
$Str_Test="This function is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation in any version
of the License."."<br>"."This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE."."<br>"."Hello Aitor, Wellcome Home"."<br>";
ECHO $Str_Test."<br>";
$Str_Test2 = ENCRYPT_DECRYPT($Str_Test);
ECHO $Str_Test2."<br><br>";
$Str_Test3 = ENCRYPT_DECRYPT($Str_Test2);
ECHO "<br>".$Str_Test3."<br>";

?>