<?php
$host = 'localhost';
    $db   = '';
    $user = '';
    $pass = ';
    $charset = 'utf8';

    $dsn = "mysql:host=$host;dbname=$db;charset=$charset";
    $opt = [
        PDO::ATTR_ERRMODE            => PDO::ERRMODE_EXCEPTION,
        PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
        PDO::ATTR_EMULATE_PREPARES   => false,
    ];
    $pdo = new PDO($dsn, $user, $pass, $opt);
    $stmt = $pdo->query('SELECT MAX(id) FROM users');
while ($row = $stmt->fetch()) {
    echo $row['id'];
}
?>