Why can't I get the last insert ID in PHP and MySQL?

Question

I have a function that creates a course. I am trying to get the Last Insert ID but it doesn't work:

public  function createCourse()
{
    require "/mysqli_connect.php";
    $course_q = "INSERT INTO course (....) VALUES (.....)";

    $course_r = mysqli_query($mysqli, $course_q);
    $course_n = mysqli_affected_rows($mysqli);
    if($course_n == 1)
    {
        mysqli_close($mysqli);
        return true;
    }
    mysqli_close($mysqli);
    return false;
}

This is the function to retrieve the last insert ID that I created in the same class as the function createCourse:

public function getLastInsertID()
{
    require "/../mysqli_connect.php";

    $course_q= "SELECT LAST_INSERT_ID()";

    $course_r = mysqli_query($mysqli, $course_q);
    $course_n = mysqli_num_rows($course_r);

    //var_dump($course_n);
    if($course_n)
    {
        $c = mysqli_fetch_assoc($course_r);
        mysqli_close($mysqli);

        return $c;
    }
    mysqli_close($mysqli);
    return NULL;
}   

This is how I call the functions:

require "/mysqli_connect.php";
$course = new Course();
$c = $course->createCourse();
$id = $course->getLastInsertID();
var_dump($id);

"$id" is always "int(0)"

I've also tried:

require "/mysqli_connect.php";
$course = new Course();
$c = $course->createCourse();
**$id = mysqli_insert_id($mysqli);**

and I've also tried:

$course_q= "SELECT LAST_INSERT_ID() from course";

but that doesn't work as well. Can you guys see what the problem is? :( The function createCourse itself is fine. It creates what I need and it's there in the database but I can't get the last insert id.

Answer 1

Although the proper way to retrieve the insert id with MySQLi is to use mysqli_insert_id(), since you're doing an associative fetch, you would need a column alias for the LAST_INSERT_ID()

 $course_q= "SELECT LAST_INSERT_ID() AS insert_id";

 // Later...
 $c = mysqli_fetch_assoc($course_r);
 echo $c['insert_id'];

However, that isn't going to work because you have already closed the connection with mysqli_close() in your createCourse() function. Instead, get the insert id inside createCourse() and return it.

public  function createCourse()
{
    // Note: You should not establish the connection in each function call.
    // it only needs to be done once per script, and you can pass the connection
    // into the class constructor or into methods that use it.
    require "/mysqli_connect.php";
    $course_q = "INSERT INTO course (....) VALUES (.....)";

    $course_r = mysqli_query($mysqli, $course_q);
    $course_n = mysqli_affected_rows($mysqli);
    if($course_n == 1)
    {
        // Get the insert id before closing the connection
        $insert_id = mysqli_insert_id($mysqli);


        // Note that there probably isn't a good reason to explicitly close the
        // connection here.  It will be closed when the script terminates.
        mysqli_close($mysqli);

        // And return it.
        return $insert_id;
    }
    mysqli_close($mysqli);
    return false;
}

require "/mysqli_connect.php";
$course = new Course();
$c = $course->createCourse();
echo $c;
// $c contains the id

Design classes to use one common connection:

The class receives a connection in its constructor.

// Make classes accept the connection as a param:
class MyClass
{
  // property to hold the connection object
 public $db;

  // constructor receives the connection
  public function __construct($connection) {
    $this->db = $connection;
  }
  
  // Methods call it as $this->db
  public function createCourse() {
    $course_r = mysqli_query($this->db, $course_q);
  }
      
}

Include the connection only once on the controlling script

require_once("/mysqli_connect.php");
// $mysqli is now defined...

// Pass it into the constructor
$course = new MyClass($mysqli);

Answer 2

The issue is you have multiple connections to the database, and last inserted ID is tracked on a per-connection basis.

Note: You should not connect to the same database every time you run a query. Simply connect at the beginning of the script, and close at the end. Connecting and disconnecting constantly will cause your script to run extremely slowly, especially with more queries.

Also, the last inserted ID is available via MySQLi's mysqli_insert_id() function.

Answer 3

Most likely because you're closing the mysqli connection at the end of createCourse().

Either don't make createCourse() responsible for closing the connection (move it elsewhere) or have createCourse() return the insert ID on success (or false on failure)