Here is my php to vb.net

PHP code:

function encrypt($string, $key) {
                        $result = '';
                        for($i=0; $i<strlen($string); $i++) {
                                    $char = substr($string, $i, 1);
                                    $keychar = substr($key, ($i % strlen($key))-1, 1);
                                    $char = chr(ord($char)+ord($keychar));
                                    $result.=$char;
                        }
                        return base64_encode($result);
            } 

VB.NET code:

Function crypter(ByVal strPass As String, ByVal key As String)
        Dim result As String = Nothing
        Dim i As Integer
        For i = 0 To strPass.Length - 1
            Dim charPass As Char
            charPass = strPass.Substring(i, 1)
            Dim keychar As Char
            keychar = key.Substring((i Mod key.Length) - 1, 1)
            charPass = Chr(Asc(charPass) + Asc(keychar))
            result += charPass
        Next [i]
        Dim Buffer As Byte() = ASCIIEncoding.ASCII.GetBytes(result)
        Return System.Convert.FromBase64String(result)

    End Function

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 At line keychar = key.Substring((i Mod key.Length) - 1, 1) i get this error: StartIndex cannot be less than zero.

my string is 13 characters and my key is about 34, can someone help with this solution? 

If i = 0, then 0 MOD anything is still zero.  Zero - one = -1, thus less than zero.

It isn't the 0 that screws up the code, it's the -1.

As for the PHP question, I'm not totally sure because I'm not a PHP guy (I could be in a day or less if need be, but there is no need). There is another post that can possibly answer your question: http://bytes.com/forum/thread163251.html