The lifecycle of an object in PHP - php

Question

I have two PHP scripts that I included below. Both of them attempt to do the same thing, but one works and one does not. I'm looking for someone to explain what PHP is doing under the covers. I'm new to PHP and I suspect that my Java experience is poisoning my thought process when I work in PHP.
What I'm attempting to do is functionally very simple -- Insert a question into a mySQL database table, retrieve the primary key of the inserted row, and then insert five answers into another table with a foreign key relationship to the question.
My original logic looked like this:
ManageQuestions.php:
<?php
session_start();
include('query.php');
echo "begin <br>";
if (isset($_POST['submit'])) {
echo "manageQuestion <br>";
$query = new Query;
$query->createTransaction();
$query->executeCreateUpdateDelete("INSERT INTO question (question) VALUES ('".$_POST['question']."'); ");
$question_pid = $query->getLastInsertedId();
$query->commitTransaction(); // Need to figure out how to do dirty reads so I can remove this.
echo $question_pid."<br>";
$result = $query->executeRead("SELECT question_pid FROM question where question_pid = '".$question_pid."';");
echo count($result)."<br>";
//if (count($result) === 1) {
$query->createTransaction(); // Need to figure out how to do dirty reads so I can remove this.
foreach($_POST['answer'] as $answer) {
$correctAnswers = 0;
$query->executeCreateUpdateDelete("INSERT INTO answer (question_fid, answer, isCorrect) VALUES ('".$question_pid."','".$answer['answer']."','".$answer['isCorrect']."')");
if ($answer['isCorrect'] === 1) {
$correctAnswers = $correctAnaswers + 1;
if ($correctAnswers > 1){
echo "Failed to insert answers";
$query->rollBackTransaction();
break;
}
}
}
echo "Success";
$query->commitTransaction();
/* } else {
echo "Failed to insert question";
$query->rollBackTransaction();
} */
}
?>
Query.php:
<?php
session_start();
class Query
{
private $host="<censored>";
private $username="<censored>";
private $password="<censored>";
private $db_name="<censored>";
private $pdo;
private $pdo_statement;
private $pdo_exception;
public function executeCreateUpdateDelete($pQuery)
{
$this->pdo_statement = $this->pdo->prepare($pQuery);
return $this->pdo_statement->execute();
}
public function executeRead($pQuery)
{
try
{
$dbh = new PDO("mysql:host=$this->host;dbname=$this->db_name", $this->username, $this->password);
$result = $dbh->query($pQuery);
$dbh = null;
return $result->fetchAll();
}
catch(PDOException $e)
{
echo $e->getMessage();
}
}
public function createTransaction()
{
$this->pdo = new PDO("mysql:host=$this->host;dbname=$this->db_name", $this->username, $this->password);
$this->pdo->beginTransaction();
}
public function commitTransaction()
{
$this->pdo->commit();
}
public function rollBackTransaction()
{
$this->pdo->rollBack();
}
public function getLastInsertedId()
{
$this->pdo->lastInsertId();
}
}
?>
When I rewrote my logic to not use a separate query class, I was able to do what I wanted to do. The only thing I've been able to find online about the life cycle of a PHP object is that it begins at the start of a script and ends at the end of a script. Does that imply that my query object is instantiated every time I call one of its methods and garbage collected when that particular method ends? Moving the logic out of that class and into the script caused my logic to work. This is what it looks like now:
ManageQuestions.php:
<?php
session_start();
include('query.php');
echo "Begin <br>";
if (isset($_POST['submit'])) {
echo "manageQuestion <br>";
$host="<censored>";
$username="<censored>";
$password="<censored>";
$db_name="<censored>";
$pdo = new PDO("mysql:host=$host;dbname=$db_name", $username, $password);
$stmt = $pdo->prepare("INSERT INTO question (question) VALUES ('".$_POST['question']."'); ");
$stmt->execute();
$question_pid = $pdo->lastInsertId();
echo $question_pid."<br>";
$stmt = $pdo->query("SELECT question_pid FROM question where question_pid = '".$question_pid."';");
$result = $stmt->fetchAll();
echo count($result)."<br>";
foreach($_POST['answer'] as $answer) {
$correctAnswers = 0;
$stmt = $pdo->prepare("INSERT INTO answer (question_fid, answer, isCorrect) VALUES ('".$question_pid."','".$answer['answer']."','".$answer['isCorrect']."')");
$stmt->execute();
}
echo "Success";
}
?>
Even though this fixed my issue, I don't understand why. If someone could explain that, I would be extremely grateful.
Cheers!

Answer 1

Does that imply that my query object is instantiated every time I call one of its methods and garbage collected when that particular method ends?
No. It's per request, not per method call. So the query object is instantiated every time the script is called and it gets unset (and not necessarily garbage collected) when the script ends.
However you could better manage the resource of the PDO object inside your Query class because you create a new instance (which would mean that it connects again to the database server which is not that cheap). So some lazy loading does not seem bad:
class Query
{
...
/** #var PDO */
private $pdo;
...
private function getPdo() {
if (!$this->pdo) {
$this->pdo = new PDO("mysql:host=$this->host;dbname=$this->db_name", $this->username, $this->password);
}
return $this->pdo;
}
public function executeRead($pQuery)
{
try {
$dbh = $this->getPdo();
$result = $dbh->query($pQuery);
return $result->fetchAll();
} catch (PDOException $e) {
echo $e->getMessage();
}
}
public function createTransaction()
{
$this->getPdo()->beginTransaction();
}
...