在这里我可以看到双方的问题。

Here I can see problems on both sides.

请记住，当编码是什么，你得到不串，而是一个字节数组。
因此，在PHP，你不需要urlen code密文。

Please keep in mind that what you get when encoding is not string, but rather an array of bytes. So in PHP you don't need to urlencode cyphertext.

base64编码是你所需要的。当您打开 base64_en code有助于你看

base64 encoding is all you need. When you open base64_encode help you see

base64_en code 恩codeS给定的使用Base64数据。这种编码是
  旨在使二进制数据生存运输

base64_encode Encodes the given data with base64. This encoding is designed to make binary data survive transport

还有一件事 - 在.NET中的信息去codeD具有正确的长度，你有填充字符来手动添加它。为RijndaelManaged的默认填充模式是PKCS7，让'坚持下去。你必须扩展您的源字符串到连块与字符code等于填充字节数。

One more thing - to have your message decoded in .net with a correct length, you have to manually append it with padding characters. Default padding mode for RijndaelManaged is PKCS7, lets' stick with it. You have to extend your source string to even blocks with characters code equal to number of padding bytes.

<?php
  $iv = "45287112549354892144548565456541";
  $key = "anjueolkdiwpoida";
  $text = "This is my encrypted message";

  // to append string with trailing characters as for PKCS7 padding scheme
  $block = mcrypt_get_block_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CBC);
  $padding = $block - (strlen($text) % $block);
  $text .= str_repeat(chr($padding), $padding);

  $crypttext = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $key, $text, MCRYPT_MODE_CBC, $iv);

  // this is not needed here            
  //$crypttext = urlencode($crypttext);

  $crypttext64=base64_encode($crypttext);
  print($crypttext64) . "\n<br/>";
?>

在C＃侧您从的base64 铸造为字节[] 为字符串为字节[] 。你所要做的，从的base64 第一次转换为字节[] 只。请记住，Base64是抱着cyphered文本是二进制数据，不串。
同时请注意，RijndaelManaged的是IDisposable的，所以我用（）构造包裹它。调用close（）是必要的，但还不够在MSDN说明。

At C# side you have casting from base64 to byte[] to string to byte[]. You have to do the first conversion from base64 to byte[] only. Remember, base64 is holding the cyphered text that is binary data, not string. Also please note that RijndaelManaged is IDisposable, so I have wrapped it in using() construct. Calling Close() is necessary but not enough as stated in MSDN.

public byte[] Decode(string str)
{
    var decbuff = Convert.FromBase64String(str);
    return decbuff;
}

static public String DecryptRJ256(byte[] cypher, string KeyString, string IVString)
{
    var sRet = "";

    var encoding = new UTF8Encoding();
    var Key = encoding.GetBytes(KeyString);
    var IV = encoding.GetBytes(IVString);

    using (var rj = new RijndaelManaged())
    {
        try
        {
            rj.Padding = PaddingMode.PKCS7;
            rj.Mode = CipherMode.CBC;
            rj.KeySize = 256;
            rj.BlockSize = 256;
            rj.Key = Key;
            rj.IV = IV;
            var ms = new MemoryStream(cypher);

            using (var cs = new CryptoStream(ms, rj.CreateDecryptor(Key, IV), CryptoStreamMode.Read))
            {
                using (var sr = new StreamReader(cs))
                {
                    sRet = sr.ReadLine();
                }
            }
        }
        finally
        {
            rj.Clear();
        }
    }

    return sRet;
}

其结果是，在C＃以下code将返回初始的字符串：

As a result, following code in C# will return you the initial string:

var iv = "45287112549354892144548565456541";
var key = "anjueolkdiwpoida";
var cypher = "u+rIlHB/2rrT/u/qFInnlEkg2unhizsNzGVb9O54sP8=";

var temp = DecryptRJ256(Decode(cypher), key, iv);