Bitwise Shift - Getting different results in C# than in php

I'm trying to convert some PHP into C# but the bitwise functions are giving me different results.

PHP will return 248

protected function readInt8()
{
    $ret = 0;
    if (strlen($this->_input) >= 1)
    {
        $sbstr = substr($this->_input, 0, 1);
        $ret = ord($sbstr);
        $this->_input = substr($this->_input, 1);
    }
    return $ret;
}

C# will return 63

private int ReadInt8()
{
    int ret = 0;
    if (input.Length >= 1)
    {
        string substr = input.Substring(0, 1);
        ASCIIEncoding ascii = new ASCIIEncoding();
        byte[] buffer = ascii.GetBytes(substr);
        ret = buffer[0]; // 63

        this.input = this.input.Substring(1);
    }

    return ret;
}

or it will return 14337

private int ReadInt8()
{
    int ret = 0;

    if (input.Length >= 1)
    {
        string substr = input.Substring(0, 1);

        ret = (int)(substr[0]); // 14337
        this.input = this.input.Substring(1);
    }

    return ret;
}

An other question here worked with the bigger values but it doesn't work with the smaller values. I would like to know what the problem is.

I'm sorry. It was a bit late yesterday.

The input converted with the function below = "Ԁϸ㠁锂Ǹϸ붻ªȁ";

public string GetString(byte[] bytes)
{
    char[] chars = new char[bytes.Length / sizeof(char)];
    System.Buffer.BlockCopy(bytes, 0, chars, 0, bytes.Length);
    return new string(chars);
}

About the shift. I thought it maybe needed a shift because the ReadInt16() needed it.

private int ReadInt16()
{
    int ret = 0;
    if (input.Length >= 2)
    {
        ret  = ((int)(this.input.Substring(0, 1)[0]) & 0xffff) >> 8;
        ret |= ((int)(this.input.Substring(1, 1)[0]) & 0x0000) >> 0;
        this.input = input.Substring(2);
    }
    return ret;
 }

I should say. It is possible that I misinterpreted the use of the function in PHP.