Users.php

<?php

require_once './vendor/autoload.php';

use Kreait\Firebase\Factory;
use Kreait\Firebase\ServiceAccount;

class Users {
    protected $database;
    protected $dbname = 'users';

    public function __construct(){
        $acc = ServiceAccount::fromJsonFile(__DIR__ . '/secret/arthursystems-php-tutorials-0066e2bd5954.json');
        $firebase = (new Factory)->withServiceAccount($acc)->create();

        $this->database = $firebase->getDatabase();
    }

    public function get(int $userID = NULL){
        if (empty($userID) || !isset($userID)) { return FALSE; }

        if ($this->database->getReference($this->dbname)->getSnapshot()->hasChild($userID)){
            return $this->database->getReference($this->dbname)->getChild($userID)->getValue();
        } else {
            return FALSE;
        }
    }

    public function insert(array $data) {
        if (empty($data) || !isset($data)) { return FALSE; }

        foreach ($data as $key => $value){
            $this->database->getReference()->getChild($this->dbname)->getChild($key)->set($value);
        }

        return TRUE;
    }

    public function delete(int $userID) {
        if (empty($userID) || !isset($userID)) { return FALSE; }

        if ($this->database->getReference($this->dbname)->getSnapshot()->hasChild($userID)){
            $this->database->getReference($this->dbname)->getChild($userID)->remove();
            return TRUE;
        } else {
            return FALSE;
        }
    }
}

$users = new Users();

//var_dump($users->insert([
//    '1' => 'John',
//    '2' => 'Doe',
//    '3' => 'Smith'
//]));

//var_dump($users->get(1));

//var_dump($users->delete(2));

var_dump($users->insert([
    '1' => 'John Doe',
]));