问如何利用md5技术使用盐

EN

<?php
require("common.php");
if (! empty ( $_POST )) {
    if (empty ( $_POST ['username'] )) {
        die ( "Please enter a username." );
    }
    if (empty ( $_POST ['password'] )) {
        die ( "Please enter a password." );
    }
    $query = "SELECT 1 FROM User WHERE username = :username";
    $query_params = array (
            ':username' => $_POST ['username'] 
    );
    try {
        $stmt = $db->prepare ( $query );
        $result = $stmt->execute ( $query_params );
    } catch ( PDOException $ex ) {
        die ( "Failed to run query: " . $ex->getMessage () );
    }
    $row = $stmt->fetch ();
    if ($row) {
        die ( "This user name is already registered" );
    }
    $password = md5($_POST['password']);
    $query = "INSERT INTO User (username, password) VALUES (:username, :password)";
    $query_params = array (
            ':username' => $_POST ['username'],
            ':password' => $password 
    );
    try {
        $stmt = $db->prepare ( $query );
        $result = $stmt->execute ( $query_params );
    } catch ( PDOException $ex ) {
        die ( "Failed to run query: " . $ex->getMessage () );
    }
    header ( "Location: login.php" );
    die ( "Redirecting to login.php" );
}
?>