php - Fatal error: Uncaught Error: Class 'KreaitFirebaseServiceAccount' not found in

I have to learn just now write mobile app web service via fire base. I followed this link: https://firebase-php.readthedocs.io/en/stable/

In my core website i create web service folder and then create my fire.php file.This file code here,

<?php

require __DIR__.'/vendor/autoload.php';

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

$serviceAccount = ServiceAccount::fromJsonFile(__DIR__.'/google-service-account.json');
$apiKey = 'AIzaSyC_vb5G9qs3NJsywbR34el1RaPj2HDhwNg';

$firebase = (new Factory)
    ->withServiceAccountAndApiKey($serviceAccount, $apiKey)
    ->withDatabaseUri('https://workarea-cb10b.firebaseio.com')
    ->create();

$database = $firebase->getDatabase();

$newPost = $database
    ->getReference('blog/posts')
    ->push([
        'title' => 'Post title',
        'body' => 'This should probably be longer.'
    ]);

$newPost->getKey(); // => -KVr5eu8gcTv7_AHb-3-
$newPost->getUri(); // => https://my-project.firebaseio.com/blog/posts/-KVr5eu8gcTv7_AHb-3-

$newPost->getChild('title')->set('Changed post title');
$newPost->getValue(); // Fetches the data from the realtime database
$newPost->remove();
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I have to call my support file here: https://github.com/kreait/firebase-php/

But still I got a:

Fatal error: Uncaught Error: Class 'Kreait\Firebase\ServiceAccount' not found in /opt/lampp/htdocs/workarea/webservice/firebase/fire.php:13 Stack trace: #0 {main} thrown in /opt/lampp/htdocs/workarea/webservice/firebase/fire.php on line 13

issue have to fix this issue. Kindly check and help me.

Mycomposer.json file

{
    "name": "kreait/firebase-php",
    "description": "Firebase Admin SDK",
    "keywords": ["firebase", "google", "sdk", "api", "database"],
    "homepage": "https://github.com/kreait/firebase-php",
    "license": "MIT",
    "authors": [
        { "name": "Jérôme Gamez", "homepage": "https://github.com/jeromegamez" }
    ],
    "require": {
        "php": "^7.0",
        "ext-mbstring": "*",
        "ext-openssl": "*",
        "fig/http-message-util": "^1.1",
        "google/auth": "^0.11.0|^1.0",
        "guzzlehttp/guzzle": "^6.2.1",
        "kreait/firebase-tokens": "^1.1.1",
        "lcobucci/jwt": "^3.2",
        "mtdowling/jmespath.php": "^2.3"
    },
    "require-dev": {
        "friendsofphp/php-cs-fixer": "^2.0",
        "phpstan/phpstan-phpunit": "^0.9.2",
        "phpunit/phpunit": "^6.0"
    },
    "autoload": {
        "psr-4": {
            "Kreait\\": "src"
        }
    },
    "autoload-dev": {
        "psr-4": {
            "Kreait\\Tests\\": "tests"
        }
    },
    "config": {
        "platform": {
            "php": "7.0"
        },
        "sort-packages": true
    },
    "extra": {
        "branch-alias": {
            "dev-master": "3.x-dev"
        }
    }
}
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