Gmap. Adding few markers (with InfoWindow)

map.php

<div id="map" style="height: 500px;"></div>
<script>
	function initMap() {
		var locations = [
			['InfoWindow Content', -33.890542, 151.274856,],
			['InfoWindow Content', -33.923036, 151.259052,],
			['InfoWindow Content', -34.028249, 151.157507,],
			['InfoWindow Content', -33.800101, 151.287478,],
			['InfoWindow Content', -33.950198, 151.259302,]
		];

		var map = new google.maps.Map(document.getElementById('map'), {
			zoom: 8,
			center: {lat: -25.363, lng: 131.044},
		});

		var infowindow = new google.maps.InfoWindow();
		var marker, i;

		for (i = 0; i < locations.length; i++) {
			marker = new google.maps.Marker({
				position: new google.maps.LatLng(locations[i][1], locations[i][2]),
				map: map
			});

			google.maps.event.addListener(marker, 'click', (function(marker, i) {
				return function() {
					infowindow.setContent(locations[i][0]);
					infowindow.open(map, marker);
				}
			})(marker, i));
		}
	}
</script>
<script async defer src="https://maps.googleapis.com/maps/api/js?key=<?php echo $map_key ?>&language=en&callback=initMap"></script>