Signing of hash using ECDSA in PHP

example.php

<?php
// configure the ECC lib
if (!defined('USE_EXT')) {
    if (extension_loaded('gmp')) {
        define('USE_EXT', 'GMP');
    } else if(extension_loaded('bcmath')) {
        define('USE_EXT', 'BCMATH');
    } else {
        die('GMP or bcmath required. (GMP is faster).');
    }
}
define('MAX_BASE', 256); // so we can use bcmath_Utils::bin2bc with "base256"
$secp256k1 = new CurveFp(
    '115792089237316195423570985008687907853269984665640564039457584007908834671663',
    '0', '7');
$secp256k1_G = new Point($secp256k1,
    '55066263022277343669578718895168534326250603453777594175500187360389116729240',
    '32670510020758816978083085130507043184471273380659243275938904335757337482424',
    '115792089237316195423570985008687907852837564279074904382605163141518161494337');

/* The hash I want to sign. */
$txHash  = '9302bda273a887cb40c13e02a50b4071a31fd3aae3ae04021b0b843dd61ad18e';

/* Private key to sign the hash with. */
$privKey = '18E14A7B6A307F426A94F8114701E7C8E774E7F9A47E2C2035DB29A206321725';

/* This is my first problem. I assume using $secp256k1_G is correct, and I assume I have
   to convert the private key to something that the point class constructer will understand.
   But what? */
$pubKey = new PublicKey($secp256k1_G, new Point('do something with $privKey I assume'));

/* Next problem, what is this secret multiplier supposed to be? */
$prvKey = new PrivateKey($pubKey, $someSecretMultiplier);

/* Last problem (I hope), $k is supposed to be between 0 and the order of the finite field the curve
   is defined on. I'm not a mathematician, so I'm not sure what to do there. */ 
$k = rand(0, 8);
$signature = $prvKey->sign($txHash, $k);