Identify the database connection code:

First of all you have to find the part of your PHP code that connects to the database using PDO.

Extract the database connection logic:

Separate the database connection logic into a separate file or class.

Create a database class:

Create a new class that encapsulates the database connection logic.

Add database methods:

Add methods to the class for performing common database operations like selecting, inserting, updating, and deleting data.

Use the database class:

Create the database class and use its methods to interact with the database.

Here's an example of how you can extract a database class using PDO:

// database.php

class Database {
  public $con;

  public function __construct($dsn, $username, $password) {
    $this->con = new PDO($dsn, $username, $password);
  }

  public function select($query) {
    $stmt = $this->con->prepare($query);
    $stmt->execute();
    return $stmt->fetchAll();
  }

  public function insert($query) {
    $stmt = $this->conn->prepare($query);
    $stmt->execute();
    return $this->conn->lastInsertId();
  }
}

Then, you can use the database class in your main code like this:

// main.php

require 'database.php';

$db = new Database('mysql:host=localhost;dbname=myapp', 'username', 'password');

$result = $db->select('SELECT * FROM applicants');

By extracting the database class, you can keep your database logic organized and reusable, making it easier to maintain.