Do I need to prepare statement on every open or can prepare only once in PDO PHP SQLite3

I found this example on net for inserting in SQLite usong PHP PDO

# no placeholders - ripe for SQL Injection!  
$STH = $DBH->("INSERT INTO folks (name, addr, city) values ($name, $addr, $city)");  

# unnamed placeholders  
$STH = $DBH->("INSERT INTO folks (name, addr, city) values (?, ?, 开发者_开发知识库?); 

# named placeholders 
$STH = $DBH->("INSERT INTO folks (name, addr, city) value (:name, :addr, :city)"); 

I am closing connection after every command ( another app only reads and mine only inserts). Do I need to prepare statement on every open or can prepare only once ?

$userprep = $userdbc->prepare("
    INSERT INTO Users (userID, userPass, userName, userEmail)
    VALUES (:userID, :userPass, :userName, :userEmail)
");
$userprep->execute($newUser);

heres an example of my code to insert a new user using placeholders..

$newUser is an associative array.

once you have prepared a statement you can execute that statement as many times as you wish, for example if i had a nested array of users to enter i could just run this:

for($i = 0; $i < count($newUser); $i++){
    $userprep->execute($newUser[$i]);
}

(the nested arrays would have to of course use associative keys)

Hope this is of some help..