PHP Rest API – get header parameter

I have created a basic rest api using php which is working.

I want to add a header customer_id to the request

When I do this I’m not seeing it output in the headers of the api call

$header = apache_request_headers();
$AuthKey=$header['Auth-Api-Key'];  
$customerId=$header['customer_id'];

If I print_r($AuthKey) as a test I can see the Auth-Api-Key value. However print_r($customerId) is blank

enter image description here