What is the correct way to configure my classes in OOP programming in PHP?

Let me start with a little introduction. I am studying OOP in PHP. I also explored design patterns, but did not yet fully understand the various types of concepts. I am at a stage where every few months I understand that I am not doing anything right and must change my style. This is so frustrating. Therefore, I would like to find the right way to do things once and for all. I tried to fully read Stackoverflow on the following topics:

ORM
Data Map
Singleton
Globals - Evil
All Related

However, I still do not quite understand some things. I post my code here in a clear and concise way and I hope that people can point out both good practices and bad ones. I will list all my questions at the end.

Please do not close the duplicate, I honestly searched almost every question on this topic, but I still want to know a few things that I could not clarify. Sorry it took so long, but I tried to organize it so that it reads well!

I will start by publishing the basics of my database class.

database.php

<?php 


class DatabaseMySQL{

    private static $dbh;

    public function __construct(){
        $this->open_connection();
    }

    public function open_connection(){
        if(!self::$dbh){
            return (self::$dbh = new PDO(DB_TYPE.':host='.DB_HOST.';dbname='.DB_NAME, DB_USER,DB_PASSWORD)) ? true : false;
        }
        return true;
    }

    public function query($sql, $params=array()){
        $this->last_query = $sql;
        $stmt = self::$dbh->prepare($sql);
        $result = $stmt->execute($params);  
        return $result ? $stmt : $stmt->errorInfo();
    }

    public function fetch_all($results, $class_name=''){
        return $results->fetchAll(PDO::FETCH_CLASS, $class_name);
    }

}

?>

. , , PDO, Static . , PDO, .

, . MainModel. , .

MainModel.php

<?php



abstract class MainModel{

    protected static $table;

    public function __construct($array=array()){
        $this->assign_known_properties($array);
    }

    public function assign_known_properties($array){
        foreach($array as $key=>$value){
            $this->$key = $value;
        }
    }

    public static function find_by_id($id){
        $db = new DatabaseMySQL();
        self::intialise_table_name();
        $id = (int) $id;
        $sql = "SELECT * FROM ".static::$table." ";
        $sql .= "WHERE id = {$id} ";    
        $result = self::find_by_sql($sql);      
        return array_shift($result);
    }

    public static function find_all(){
        $db = new DatabaseMySQL();
        self::intialise_table_name();
        $sql = "SELECT * FROM ".self::$table." ";
        return self::find_by_sql($sql);
    }

    public static function fetch_as_objects($results){
        $db = new DatabaseMySQL();
        $called_class = get_called_class();
        $results = $db->fetch_all($results, $called_class);
        return $results;
    }

    public static function find_by_sql($sql){
        $db = new DatabaseMySQL();
        $results = $db->query($sql);
        return $results ? self::fetch_as_objects($results) : false; 
    }

    public static function intialise_table_name(){
        $called_class = get_called_class();
        static::$table = strtolower($called_class).'s';
    }

    public function get_table_fields(){
        self::intialise_table_name();
        $sql = "SHOW FIELDS FROM ".static::$table." ";
        return self::find_by_sql($sql);
    }

    public function set_table_details(){
        $fields = $this->get_table_fields();
        $total = count($fields);
        $array = array();
        foreach($fields as $object){
            $array [] = $object->Field;
        }
        $this->table_details = array('objects'=>$fields,'array'=>$array,'total'=>$total);
        $this->set_placeholders_for_new_record();
        $this->set_properties_as_array();
        $this->set_properties_as_array(true);
    }

    public function set_properties_as_array($assoc=false){
        $array = array();
        if (!$assoc){
            foreach($this->table_details['array'] as $field){
                if(isset($this->$field)){
                    $array [] = $this->$field;
                }else{
                    $array [] = NULL;
                }
            }
            $this->table_details['values'] = $array;
        }else{
            foreach($this->table_details['array'] as $field){
                if(isset($this->$field)){
                    $array[$field] = $this->$field;
                }else{
                    $array [$field] = NULL;
                }
            }
            $this->table_details['assoc_values'] = $array;
        }
    }

    public function set_placeholders_for_new_record(){
        $string = '';
        for($i=0; $i<$this->table_details['total']; $i++){
            $string .= '? ';
            if(($i+1) != $this->table_details['total'] ){
                $string .= ", ";
            }
        }
        $this->table_details['placeholders'] = $string;
    }

    public function create(){
        $db = new DatabaseMySQL();
        $this->set_table_details();
        $sql = "INSERT INTO ".static::$table." ";
        $sql .= " VALUES({$this->table_details['placeholders']}) ";
        $result = $db->query($sql, $this->table_details['values']);

        // If array is returned then there was an error.
        return is_array($result) ? $result : $db->insert_id();
    }

    public function update(){
        $db = new DatabaseMySQL();
        $this->set_table_details();
        $sql = "UPDATE ".static::$table." ";
        $sql .= " SET ";
            $count = 1;
            foreach($this->table_details['array'] as $field){
                $sql .= "{$field} = :{$field} ";
                if($count < $this->table_details['total']){
                    $sql .= ", ";
                }
                $count++;
            }

        $sql .= " WHERE id = {$this->id} ";
        $sql .= " LIMIT 1 ";
        $result = $db->query($sql, $this->table_details['assoc_values']);
        return $result;
    }

    public function save(){
        return isset($this->id) ? $this->update() : $this->create();
    }
}


?>

. , find_by_id($int), . Late Static Bindings, , $stmt->fetchAll(PDO::FETCH_CLASS, $class_name) , .

DatabaseMySQL. $table, SQL- , s. , User, users.

, . , , . .

User.php

class User extends MainModel{

}

Question.php

class Question extends MainModel{

}

, , . :

$user = new User(array('username'=>'John Doe'));
echo $user->username; // prints *John Doe*
$user->save(); // saves (or updates) the user into the database.

.

$user = User::find_by_id(1);
echo $user->username; // prints users name

, :

• 1) ( ). Data Mapper? ? ( )? ?

• 2) , ?

• 3) , , , ,

• 4) , , , , ?

• 5) , , , ?

• 6) create, update, delete, , , find_by_id, find_all, . , ?

• 7) $load->('UserClass') , mappers, ?