$ db-> prepare（$ sql）在PHP中做什么？

Question

有人可以解释以下代码行吗：

$statement = $db->prepare($sqlInsert);

从下面的代码中：

<?php

include_once 'resource/Database.php';
var_dump($_POST);

if (isset($_POST['email'])) {
    $email    = $_POST['email'];
    $username = $_POST['username'];
    $password = $_POST['password'];

    $sqlInsert = "INSERT INTO users (username, email, password, join_date)
                        VALUES (:username, :email, :password, now())";

    //Sanitize whatever input users entered (advantage of PDO)
    $statement = $db->prepare($sqlInsert);
}
?>