<?PHP 
 
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&#65533;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."; 
 
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>"; 
 
?>