Don'；我不太了解SQL注入

我读了很多关于sql注入的书，我了解它是如何导致问题的（例如：DROP TABLE__etc）。但我不确定我所遵循的教程是如何防止这种情况发生的。我只是在学习PDO，我想我理解它。

此代码不受SQL注入的影响吗为什么？（使用这些准备好的语句需要做更多的工作，所以我想确保我不是在浪费时间-如果代码可以改进，请告诉我！）

$conn = new PDO("mysql:host=$DB_HOST;dbname=$DB_DATABASE",$DB_USER,$DB_PASSWORD);
// Get the data
$firstname = $_POST["v_firstname"];
$lastname =  $_POST["v_lastname"];
$origincountry =  $_POST["v_origincountry"];
$citizenship = $_POST["v_citizenship"];
$gender = $_POST["v_gender"];
$dob = $_POST["v_dob"];
$language = $_POST["v_language"];
$landing = $_POST["v_landing"];
$email = $_POST["v_email"];
$phone = $_POST["v_phone"];
$cellphone = $_POST["v_cellphone"];
$caddress = $_POST["v_caddress"];
$paddress = $_POST["v_paddress"];
$school = $_POST["v_school"];
$grade = $_POST["v_grade"];
$smoker = $_POST["v_smoker"];
$referred = $_POST["v_referred"];
$notes = $_POST["v_notes"];

//Insert Data
$sql = "INSERT INTO clients (firstname, lastname, origincountry, citizenship, gender, dob, language, landing, email, phone, cellphone, caddress, paddress, school, grade, smoker, referred, notes) 
        VALUES (:firstname, :lastname, :origincountry, :citizenship, :gender, :dob, :language, :landing, :email, :phone, :cellphone, :caddress, :paddress, :school, :grade, :smoker, :referred, :notes)";
$q = $conn->prepare($sql);
$q->execute(array(':firstname'=>$firstname,
                  ':lastname'=>$lastname,
                  ':origincountry'=>$origincountry,
                  ':citizenship'=>$citizenship,
                  ':gender'=>$gender,
                  ':dob'=>$dob,
                  ':language'=>$language,
                  ':landing'=>$landing,
                  ':email'=>$email,
                  ':phone'=>$phone,
                  ':cellphone'=>$cellphone,
                  ':caddress'=>$caddress,
                  ':paddress'=>$paddress,
                  ':school'=>$school,
                  ':grade'=>$grade,
                  ':smoker'=>$smoker,
                  ':referred'=>$referred,
                  ':notes'=>$notes));