How to use Open ai Assistants key in PHP

<?php
require 'vendor/autoload.php';

use GuzzleHttpClient;

function startQuiz() {
    $client = new Client();
    $apiKey = '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'; // Replace with your actual API key

    try {
`so this is my code now how can add my assistans key`
        $response = $client->request('POST', 'https://api.openai.com/v1/chat/completions', [
            'headers' => [
                'Authorization' => 'Bearer ' . $apiKey,
                'Content-Type' => 'application/json'
            ],
            'json' => [
                'model' => 'gpt-4-1106-preview', // Your specific GPT-3.5-turbo model
                'messages' => [
                    ['role' => 'system', 'content' => 'You are a helpful assistant.'],
                    ['role' => 'user', 'content' => 'start quiz Francals Highway Code multiple choose']
                ]
            ]
        ]);

        $body = $response->getBody();
        $content = json_decode($body);
        echo $content->choices[0]->message->content;
    } catch (Exception $e) {
        echo 'Error in starting the quiz: ',  $e->getMessage(), "n";
    }
}

startQuiz();