dbh.php

<?php
// Connect to DB

class Dbh {
  private $host = "";
  private $user = "";
  private $pwd = "";
  private $dbName = "";
  
  protected function connect(){
    $dsn = 'mysql:host=' . $this->host . ';dbname=' . $this->dbName;
    $pdo = new PDO($dsn, $this->user, $this->pwd);
    
    // Set default return type
    $pdo->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC);
    
    return $pdo;
  }
}

user.php

<?php

class User extends Dbh {
  // Without prepared statements
  public function getUsers() {
    $sql = "SELECT * FROM users;";
    $stmt = $this->connect()->query($sql);
    while($row = $stmt->fetch()) {
      echo $row['first_name'];
    }
  }
  
  // With prepared statement
  public function getUsersStmt($fName) {
    $sql = "SELECT * FROM users WHERE first_name = ?;";
    $stmt = $this->connect()->prepare($sql);
    $stmt->execute([$fName]);
    $names = $stmt->fetchAll();
    foreach($names as $name) {
      echo $name['first_name'];
    }
  }
  
  // Create
  public function setUserStmt($fName, $lName) {
    $sql = "INSERT INTO users(first_name, last_name) VALUES (?, ?);";
    $stmt = $this->connect()->prepare($sql);
    $stmt->execute([$fName, $lName]);
  }
}