To set custom variables in Twilio SendGrid using the PHP SDK so that they appear in webhooks, you need to use the custom_args feature. This allows you to attach custom data to your email, which will then be included in the webhook payload when the event occurs (e.g., email opened, clicked, etc.).

Here's a step-by-step guide on how to achieve this:

1. Install the SendGrid PHP SDK: If you haven't already installed the SendGrid PHP SDK, you can do so using Composer:

   composer require sendgrid/sendgrid

2. Set Up Your SendGrid Client: Initialize the SendGrid client with your API key.

   require 'vendor/autoload.php'; // If you're using Composer (recommended)
   
   $apiKey = 'YOUR_SENDGRID_API_KEY';
   $sendgrid = new \SendGrid($apiKey);

3. Create an Email with Custom Args: When creating your email, you can add custom arguments using the addCustomArg method. These arguments will be included in the webhook payload.

   $email = new \SendGrid\Mail\Mail();
   $email->setFrom("[email protected]", "Example Sender");
   $email->setSubject("Sending with SendGrid is Fun");
   $email->addTo("[email protected]", "Example Recipient");
   $email->addContent("text/plain", "and easy to do anywhere, even with PHP");
   $email->addContent(
       "text/html", "<strong>and easy to do anywhere, even with PHP</strong>"
   );
   
   // Add custom arguments
   $email->addCustomArg("user_id", "12345");
   $email->addCustomArg("campaign", "welcome_series");
   
   try {
       $response = $sendgrid->send($email);
       echo $response->statusCode();
       print_r($response->headers());
       echo $response->body();
   } catch (Exception $e) {
       echo 'Caught exception: '. $e->getMessage() ."\n";
   }

4. Configure Webhooks: Ensure that your SendGrid account is set up to send event webhooks. You can configure this in the SendGrid dashboard under Settings > Mail Settings > Event Webhook.

5. Receive and Parse Webhooks: Your endpoint that receives the webhooks will need to parse the incoming JSON payload. The custom arguments will be included in the payload under the custom_args field.

   Example payload snippet:

   {
     "email": "[email protected]",
     "timestamp": 1609459200,
     "event": "delivered",
     "custom_args": {
       "user_id": "12345",
       "campaign": "welcome_series"
     }
   }

By following these steps, you can successfully include custom variables in your SendGrid emails and have them appear in the webhook payloads. This is particularly useful for tracking and analytics purposes.