Encryption and Decryption function in 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)
{
//Function : encrypt/decrypt a string message v.1.0 without a known key
//Author : Sumit Joshi
//Email : joshisumitnet@yahoo.com
//Date : 28-May-2009
$Len_Str_Message=STRLEN($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

//Usage of above function

$Str_Test="This is a sample text which shows an Encryped and Decrypted code.";
ECHO $Str_Test."
";
$Str_Test2 = ENCRYPT_DECRYPT($Str_Test);
ECHO $Str_Test2."

";
$Str_Test3 = ENCRYPT_DECRYPT($Str_Test2);
ECHO "
".$Str_Test3."
";
?>