Proper way to access database in PHP classes

Make a global Instance:

//db.php include once 
class DB {
  #bind connenction in it
}
#make instance
$db = new DB($config);#use for User Instances

and then:

class User {

private $db;
private $firstName;
private $familyName;
private $emailAddress;

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

public function validate($username, $password,$db) {
    //check if user with name and pass exist in DB

    //If user exist, populate member variables
    $this->emailAddress = ...

}

public function getEmail(){
    return $this->emailAddress;
}
}
$user = new User($db);

Is one way. But you telling to less about how you want to use the classes.

I would go with PHPAcademy's login/register tutorial. He have DB class that handles almost anything you need. Here is sample of his code, slightly modified by me, but all credits to Alex Garrett.

<?php

class DB {
    private static $_instance = null;
    private $_pdo,
            $_query, 
            $_error = false,
            $_results,
            $_count = 0;

    private function __construct() {
        try {
            $this->_pdo = new PDO('mysql:host=' . Config::get('mysql/host') . ';dbname=' . Config::get('mysql/db'), Config::get('mysql/username'), Config::get('mysql/password'));
        } catch(PDOException $e) {
            die($e->getMessage());
        }
    }

    public static function getInstance() {
        if(!isset(self::$_instance)) {
            self::$_instance = new DB();
        }
        return self::$_instance;
    }

    public function query($sql, $params = array()) {
        $this->_error = false;
        if($this->_query = $this->_pdo->prepare($sql)) {
            $x = 1;
            if(count($params)) {
                foreach($params as $param) {
                    if (is_int($param)) {
                        $this->_query->bindValue($x, $param, PDO::PARAM_INT);
                    } else {
                        $this->_query->bindValue($x, $param);
                    }

                    $x++;
                }
            }



            if($this->_query->execute()) {
                $this->_results = $this->_query->fetchAll(PDO::FETCH_OBJ);
                $this->_count = $this->_query->rowCount();
            }
            else {
                $this->_error = true;
                print_r($this->_query->errorInfo());
            }
        }

        return $this;
    }

    public function action($action, $table, $where = array()) {
        if(count($where) === 3){
            $operators = array('=', '>', '<', '>=', '<=', '!=');

            $field    = $where[0];
            $operator = $where[1];
            $value    = $where[2];

            if(in_array($operator, $operators)) {
                $sql = "{$action} FROM {$table} WHERE {$field} {$operator} ?";

                if(!$this->query($sql, array($value))->error()) {
                    return $this;
                }
            }
        } else if (count($where) === 0) {

            $sql       = "{$action} FROM {$table}";

            if(!$this->query($sql)->error()) {
                    return $this;
                }

        }



        return false;
    }

    public function get($table, $where) {
        return $this->action('SELECT *', $table, $where);
    }

    public function delete($table, $where) {
        return $this->action('DELETE', $table, $where);
    }

    public function getAll($table) {
        return $this->action('SELECT *', $table);
    }

    public function first() {
        return $this->results()[0];
    }

    public function last() {
        $i = count($this->results()) - 1;

        return $this->results()[$i];
    }

    public function insert($table, $fields = array()) {
        if(count($fields)) {
            $keys = array_keys($fields);
            $values = '';
            $x = 1;

            foreach ($fields as $field) {
                $values .= '?';
                if($x < count($fields)) {
                    $values .= ', ';
                }
                $x++;
            }

            $sql = "INSERT INTO {$table} (`"  . implode('` , `', $keys) . "`) VALUES({$values})";

            if (!$this->query($sql, $fields)->error()) {
                return true;
            }
        }

        return false;
    }

    public function update($table, $where, $parametar, $fields) {
        $set = '';
        $x = 1;

        foreach ($fields as $name => $value) {
            $set .= "{$name} = ?";
            if ($x < count($fields)) {
                $set .= ', ';
            }

            $x++;
        }
        if (is_int($parametar)) {
            $sql = "UPDATE {$table} SET {$set} WHERE {$where} = {$parametar}";
        } else {
            $sql = "UPDATE {$table} SET {$set} WHERE {$where} = '{$parametar}'";
        }

        if (!$this->query($sql, $fields)->error()) {
                return true;
            }

        return false;

    }

    public function results() {
        return $this->_results;
    }

    public function error() {
        return $this->_error;
    }

    public function count() {
        return $this->_count;
    }

}

Then you can query database like DB::getInstance()->getAll('tableName')->results();. Change DB credentials in __construct, or watch his videos (which I recomend).