Limit the number of PHP API calls

Question

I tried to create an API in PHP, I don't know if it's good, but it works.

I would like to secure the API with a key.

include 'src/callapi.php'; 
include 'src/variable.php';

$sortie = array('lat' => $lat, 'long' => $long, 'goodtosee' => $goodtosee);

echo json_encode($sortie);

I tried this to secure access:

$key = "blablabla";

if ($_GET["key"] == $key) {
    $sortie = array('lat' => $lat, 'long' => $long, 'goodtosee' => $goodtosee);
    echo json_encode($sortie);
} else {
    echo "key not valid";
}

If the key parameter in the url is equal to "blablabla" then it displays the data otherwise it displays "key not valid

How I can limit the number of calls per month?