How can I populate WhatsApp template parameters with PHP?

Hello
I am using this Github Brevo Library to send transactional emails and SMS. I would now like to use the WhatsApp feature.

I have gone through the WhatsApp setup process and am able to send messages successfully, but I am having trouble populating the parameters set out in my template.

I have created a parameter in my template: {{unitNumber}}

I run the following code which populates the number into an object with property « unitNumber »,

$params = new stdClass();
	$params->unitNumber = '357001';
	
	try {
		$sendWhatsappMessage["templateId"] = 8;
		$sendWhatsappMessage["senderNumber"] = "447870219640";
		$sendWhatsappMessage["contactNumbers"] = ["447843915605"];
		$sendWhatsappMessage["params"] = $params;


		$result = $wapInstance->sendWhatsappMessage($sendWhatsappMessage);
		print_r($result);
	} catch (Exception $e) {
		echo 'Exception when calling TransactionalWhatsAppApi->sendWhatsappMessage: ', $e->getMessage(), PHP_EOL;
	}

However when I send it, the message just shows the template field « {{unitNumber}} ».

What am I doing wrong? I have tried several ways of adding the parameters, including defining it as an array etc array(« unitNumber » => « 357001 »); but it makes so difference…