get coordinates from address with google maps api

GMGetCoordinates.php

<?php
function GMGetCoordinates($address) {
    
    $address = urlencode($address);

    $url = "http://maps.google.com/maps/api/geocode/json?address=$address&sensor=false";

    $ch = curl_init();
    $options = array(
        CURLOPT_SSL_VERIFYPEER => false,
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_URL            => $url,
        CURLOPT_HEADER         => false,
    );
        
    curl_setopt_array($ch, $options);
    $response = curl_exec($ch);
    curl_close($ch);

    if (!$response) {
        return false;
    }

    $response = json_decode($response);

    if ($response->status !== 'OK') {
        return false;
    }

    $lat  = $response->results[0]->geometry->location->lat;
    $long = $response->results[0]->geometry->location->lng;


    return "$lat, $long";
}

Its showing following error, Please inform how to fix it:-

{
"error_message" : "You must use an API key to authenticate each request to Google Maps Platform APIs. For additional information, please refer to http://g.co/dev/maps-no-account",
"results" : [],
"status" : "REQUEST_DENIED"
}