Encryption between PHP and C#

I'm trying to reproduce the same encryption algorithm used by a 3rd party on his C# based API. I need to pass a password in the URL encrypted with the same algorithm they use. Here it is the C# code:

public static class EncryptionHelper { // This constant string is used as a "salt" value for the PasswordDeriveBytes function calls. // This size of the IV (in bytes) must = (keysize / 8). Default keysize is 256, so the IV must be // 32 bytes long. Using a 16 character string here gives us 32 bytes when converted to a byte array. private const string initVector = "tu89geji340t89u2"; // This constant is used to determine the keysize of the encryption algorithm. private const int keysize = 256; /// <summary> /// Encrypts a text using a passPhrase /// </summary> /// <param name="plainText">string that contains the text to encrypt</param> /// <param name="passPhrase">string that contains the passPhrase for encryption</param> /// <returns></returns> public static string Encrypt(string plainText, string passPhrase) { byte[] initVectorBytes = Encoding.UTF8.GetBytes(initVector); byte[] plainTextBytes = Encoding.UTF8.GetBytes(plainText); PasswordDeriveBytes password = new PasswordDeriveBytes(passPhrase, null); byte[] keyBytes = password.GetBytes(keysize / 8); RijndaelManaged symmetricKey = new RijndaelManaged(); symmetricKey.Mode = CipherMode.CBC; ICryptoTransform encryptor = symmetricKey.CreateEncryptor(keyBytes, initVectorBytes); MemoryStream memoryStream = new MemoryStream(); CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write); cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length); cryptoStream.FlushFinalBlock(); byte[] cipherTextBytes = memoryStream.ToArray(); memoryStream.Close(); cryptoStream.Close(); return Convert.ToBase64String(cipherTextBytes); } /// <summary> /// Decrypts a text using a passPhrase /// </summary> /// <param name="cipherText">string that contains the encrypted text</param> /// <param name="passPhrase">string that containts the passPhrase for decryption</param> /// <returns></returns> public static string Decrypt(string cipherText, string passPhrase) { byte[] initVectorBytes = Encoding.ASCII.GetBytes(initVector); byte[] cipherTextBytes = Convert.FromBase64String(cipherText); PasswordDeriveBytes password = new PasswordDeriveBytes(passPhrase, null); byte[] keyBytes = password.GetBytes(keysize / 8); RijndaelManaged symmetricKey = new RijndaelManaged(); symmetricKey.Mode = CipherMode.CBC; ICryptoTransform decryptor = symmetricKey.CreateDecryptor(keyBytes, initVectorBytes); MemoryStream memoryStream = new MemoryStream(cipherTextBytes); CryptoStream cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read); byte[] plainTextBytes = new byte[cipherTextBytes.Length]; int decryptedByteCount = cryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length); memoryStream.Close(); cryptoStream.Close(); return Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount); } /// <summary> /// Changes the reserved path characters to avoid issues with the URL /// </summary> /// <param name="encryptedText"></param> /// <returns></returns> public static string EncodeForURL(string encryptedText) { encryptedText = encryptedText.Replace("+", "__43__"); encryptedText = encryptedText.Replace("/", "__47__"); encryptedText = encryptedText.Replace(";", "__59__"); encryptedText = encryptedText.Replace("?", "__63__"); return encryptedText; } /// <summary> /// Returns to the original characters for the encrypted text /// </summary> /// <param name="encryptedText"></param> /// <returns></returns> public static string DecodeForURL(string encryptedText) { encryptedText = encryptedText.Replace("__43__", "+"); encryptedText = encryptedText.Replace("__47__", "/"); encryptedText = encryptedText.Replace("__59__", ";"); encryptedText = encryptedText.Replace("__63__", "?"); return encryptedText; } } } 

I've tried reproducing it in PHP but I can't get the same result they are achieving with their algorithm. I've been reading other related threads like this or this but couldn't find a solution. Any help?