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

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 

?>