为什么我的PHP SHA256哈希值不等于C#SHA256管理哈希

PHP：

$hash = hash('sha256',$userData['salt'] . hash('sha256',$password) );

C#

public static string ComputeHash(string plainText,string salt)
    {
        // Convert plain text into a byte array.
        byte[] plainTextBytes = Encoding.UTF8.GetBytes(plainText);
        byte[] saltBytes = Encoding.UTF8.GetBytes(salt);

        SHA256Managed hash = new SHA256Managed();

        // Compute hash value of salt.
        byte[] plainHash = hash.ComputeHash(plainTextBytes);

        byte[] concat = new byte[plainHash.Length + saltBytes.Length];

        System.Buffer.BlockCopy(saltBytes,concat,saltBytes.Length);
        System.Buffer.BlockCopy(plainHash,saltBytes.Length,plainHash.Length);

        byte[] tHashBytes = hash.ComputeHash(concat);

        // Convert result into a base64-encoded string.
        string hashValue = Convert.ToBase64String(tHashBytes);

        // Return the result.
        return hashValue;
    }