Q:

How to send data from a mysql database to JS script?

My intention is to query an MySQL database using a PHP script and then send the information to JavaScript.
I know how to encode the info into the JSON and send the request to the server script.
How does the PHP script know it has been queried it should answer? 

A:

Thank you for all the feedback provided!
Unfortunately, I was not sure, whether I had to use exit or header('Content-Type: application/json') as the answers suggested, I decided to use both.
In the end, I added both. This is what worked:
header('Content-Type: application/json');
echo json_encode($rows, JSON_NUMERIC_CHECK);
exit;

