What are objects in php?

Question

PHP

I'm trying to make a simple website using the ActiveRecord pattern.
In the ActiveRecord and ORM pattern, we create a class and properties that correspond to a table, we get one table - one class. Plus, each class has 4 methods - insert, delete, edit, update.
When we get all the records from the database using the FETCH_CLASS method, we create an object of the desired class for each row and add this object to the array. It turns out an array of objects.
I do not quite understand what the objects in this array are? Something like an associative array, where the names of the variables in the class are the keys, and the values ​​of the variables, the values ​​of these keys?
Are they different from arrays in that certain methods can be applied to them? for example update, find the object where id = 1 and assign other values ​​to the variables.
in some example I saw that with the insert method they check if there is an object where id is already set or something like that. Why check the id if in the database the id type is SERIAL and when a new record is added it will be unique, but we do not write the id in the sql query itself?

public function insert() {

        // Есть у объекта статьи ID?
        if ( !is_null( $this->id ) ) trigger_error ( "Article::insert(): Attempt to insert an Article object that already has its ID property set (to $this->id).", E_USER_ERROR );

        // Вставляем статью
        $conn = new PDO( DB_DSN, DB_USERNAME, DB_PASSWORD );
        $sql = "INSERT INTO articles ( publicationDate, title, summary, content ) VALUES ( FROM_UNIXTIME(:publicationDate), :title, :summary, :content )";
        $st = $conn->prepare ( $sql );
        $st->bindValue( ":publicationDate", $this->publicationDate, PDO::PARAM_INT );
        $st->bindValue( ":title", $this->title, PDO::PARAM_STR );
        $st->bindValue( ":summary", $this->summary, PDO::PARAM_STR );
        $st->bindValue( ":content", $this->content, PDO::PARAM_STR );
        $st->execute();
        $this->id = $conn->lastInsertId();
        $conn = null;

        echo ' news addes';
    }

PS Are there any good materials, examples of using ActiveRecord and ORM?