how to store connection object in the constant in php

1<?php
2
3include_once('IConnectInfo.php');
4
5class Database implements IConnectInfo
6{
7
8    private static $instance = null;
9    private $conn;
10
11    private $server = IConnectInfo::HOST;
12    private $currentDB = IConnectInfo::DBNAME;
13    private $user = IConnectInfo::UNAME;
14    private $pass = IConnectInfo::PW;
15
16    private function __construct()
17    {
18        try {
19            $this->conn = new PDO("mysql:host=$this->server;dbname=$this->currentDB", $this->user, $this->pass
20            );
21            $this->conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
22            $this->conn->exec('set names utf8');
23            echo $this->server . " connected successfully" . PHP_EOL;
24        } catch (PDOException $e) {
25            echo "Connection failed: " . $e->getMessage();
26            die;
27        }
28    }
29
30    public static function getInstance()
31    {
32        if (!self::$instance) {
33            self::$instance = new Database();
34        }
35
36        return self::$instance;
37    }
38
39    public function getConnection()
40    {
41        return $this->conn;
42    }
43
44
45    public function getSelectQueryResult($query = '')
46    {
47        try {
48            $query = $this->conn->prepare($query);
49            $query->execute();
50            return $query->fetchAll(PDO::FETCH_ASSOC);
51        } catch (PDOException $e) {
52            echo $query . "<br>" . $e->getMessage();
53        }
54    }
55}
56