In our previous conversation, I asked you "Is it possible to fetch the filenames of all PDFs and their links from a specific Google Drive folder using PHP and display them on an HTML page using AJAX (XHR)?".
And following is your response:
Yes, it is possible to fetch the filenames of all PDFs and their links from a specific Google Drive folder using PHP and display them on an HTML page using AJAX (XHR). Below, I'll outline the steps you need to take to achieve this:

### **1. Set Up Google Drive API Access**

To interact with Google Drive programmatically, you'll need to use the Google Drive API.

- **Enable the Google Drive API** in your Google Cloud Console:
  - Go to the [Google Cloud Console](https://console.cloud.google.com/).
  - Create a new project or select an existing one.
  - Navigate to **APIs & Services** > **Library**.
  - Search for **Google Drive API** and enable it.
  
- **Create Credentials**:
  - Go to **APIs & Services** > **Credentials**.
  - Click **Create Credentials** and choose **OAuth client ID**.
  - Set the application type to **Web application**.
  - Provide authorized redirect URIs (e.g., `http://yourdomain.com/oauth2callback.php`).

- **Download the Credentials JSON**:
  - After creating the credentials, download the JSON file containing your client ID and client secret.

### **2. Install the Google API PHP Client Library**

Use Composer to install the Google API Client Library for PHP:

```bash
composer require google/apiclient:^2.0
```

If you're not using Composer, you can download the library from GitHub and include it manually.

### **3. Implement OAuth 2.0 Authentication in PHP**

You'll need to authenticate your PHP application to access the Google Drive API.

- **Start the OAuth Flow**:
  - Create a script that initiates the OAuth flow (`authorize.php`).

```php
<?php
require_once 'vendor/autoload.php';

session_start();

$client = new Google\Client();
$client->setAuthConfig('credentials.json');
$client->addScope(Google\Service\Drive::DRIVE_READONLY);
$client->setRedirectUri('http://' . $_SERVER['HTTP_HOST'] . '/oauth2callback.php');

$authUrl = $client->createAuthUrl();
header('Location: ' . filter_var($authUrl, FILTER_SANITIZE_URL));
```

- **Handle the OAuth Callback**:
  - Create a script to handle the OAuth callback (`oauth2callback.php`).

```php
<?php
require_once 'vendor/autoload.php';

session_start();

$client = new Google\Client();
$client->setAuthConfig('credentials.json');
$client->setRedirectUri('http://' . $_SERVER['HTTP_HOST'] . '/oauth2callback.php');
$client->addScope(Google\Service\Drive::DRIVE_READONLY);

if (!isset($_GET['code'])) {
    // Redirect to the OAuth consent page
    $authUrl = $client->createAuthUrl();
    header('Location: ' . $authUrl);
    exit;
} else {
    // Exchange authorization code for access token
    $client->authenticate($_GET['code']);
    $_SESSION['access_token'] = $client->getAccessToken();
    header('Location: list_files.php');
    exit;
}
```

### **4. Write the PHP Script to List PDF Files (`list_files.php`)**

This script will use the access token to list all PDF files in the specific Google Drive folder.

```php
<?php
require_once 'vendor/autoload.php';

session_start();

if (!isset($_SESSION['access_token']) || $_SESSION['access_token'] === null) {
    header('Location: authorize.php');
    exit;
}

$client = new Google\Client();
$client->setAccessToken($_SESSION['access_token']);

if ($client->isAccessTokenExpired()) {
    // Refresh the token if possible, else prompt for authorization again
    if ($client->getRefreshToken()) {
        $client->fetchAccessTokenWithRefreshToken($client->getRefreshToken());
        $_SESSION['access_token'] = $client->getAccessToken();
    } else {
        header('Location: authorize.php');
        exit;
    }
}

$driveService = new Google\Service\Drive($client);

// Replace with your folder ID
$folderId = 'YOUR_FOLDER_ID';

// Prepare the query to fetch PDFs from the folder
$query = sprintf(&quot;'%s' in parents and mimeType='application/pdf'&quot;, $folderId);

$parameters = [
    'q' => $query,
    'fields' => 'files(id, name, webViewLink, webContentLink)'
];

$results = $driveService->files->listFiles($parameters);

$files = [];

foreach ($results->getFiles() as $file) {
    $files[] = [
        'name' => $file->getName(),
        'id' => $file->getId(),
        'webViewLink' => $file->getWebViewLink(),
        'webContentLink' => $file->getWebContentLink()
    ];
}

// Output the data as JSON
header('Content-Type: application/json');
echo json_encode($files);
```

### **5. Create the HTML Page with AJAX Request**

Your HTML page will use JavaScript to perform an AJAX (XHR) request to `list_files.php` and display the list of PDFs.

```html
<!DOCTYPE html>
<html>
<head>
    <title>PDF List</title>
</head>
<body>
    <h1>PDF Files in Google Drive Folder</h1>
    <ul id=&quot;pdf-list&quot;></ul>

    <script>
        function fetchPDFFiles() {
            var xhr = new XMLHttpRequest();
            xhr.open('GET', 'list_files.php', true);

            xhr.onload = function () {
                if (xhr.status === 200) {
                    var files = JSON.parse(xhr.responseText);
                    var pdfList = document.getElementById('pdf-list');
                    pdfList.innerHTML = '';

                    files.forEach(function(file) {
                        var listItem = document.createElement('li');
                        var link = document.createElement('a');
                        link.textContent = file.name;
                        link.href = file.webViewLink; // or file.webContentLink for direct download
                        link.target = '_blank';
                        listItem.appendChild(link);
                        pdfList.appendChild(listItem);
                    });
                } else {
                    console.error('Failed to fetch files.');
                }
            };

            xhr.onerror = function () {
                console.error('An error occurred while making the request.');
            };

            xhr.send();
        }

        // Fetch the PDF files when the page loads
        window.onload = fetchPDFFiles;

        // Optionally, you can set an interval to refresh the list periodically
        // setInterval(fetchPDFFiles, 60000); // Refresh every 60 seconds
    </script>
</body>
</html>
```

### **6. Handle New PDFs Automatically**

Since your `list_files.php` script queries the Google Drive API every time it's called, any new PDFs added to the folder will automatically appear when you refresh the page or when the AJAX request is made.

If you want the list to update without user interaction, you can use `setInterval` in JavaScript to periodically call `fetchPDFFiles()`.

### **7. Notes and Considerations**

- **Permissions**: Ensure that the OAuth credentials have the necessary permissions to access the files in the folder.

- **Access Types**:
  - `webViewLink`: Provides a link to view the file in Google Drive (requires user to have access).
  - `webContentLink`: Provides a direct download link to the file.

- **Folder ID**: Replace `'YOUR_FOLDER_ID'` in the script with the actual ID of your Google Drive folder. You can find this ID in the URL when you open the folder in Google Drive.

- **Security**: Be cautious with OAuth tokens and client credentials. Never expose them publicly.

- **Session Management**: Adjust session handling as needed for your application.

- **CORS**: If you run into Cross-Origin Resource Sharing (CORS) issues, ensure that your PHP script sets the appropriate headers.

### **Conclusion**

By following these steps, you can fetch and display a list of PDF files from a specific Google Drive folder using PHP and AJAX. The process involves setting up the Google Drive API, handling OAuth 2.0 authentication, querying the Drive API for files, and updating the HTML page dynamically.

---

**References**:

- [Google Drive API Documentation](https://developers.google.com/drive/api/v3/about-sdk)
- [Google API Client Library for PHP](https://github.com/googleapis/google-api-php-client)

--

But it is using OAuth client ID. Can you guide me using API keys? Is it possible to use API Keys?