Adicionando vários marcadores no Google Maps do banco de dados MySQL

Será muito fácil buscar todas as latitudes e longitudes.

require_once "db/db_handle.php";
$select = "SELECT * FROM map";
$data = $db->query($select);
foreach ($data as $key)
            $locations[]=array( 'name'=>'Location Name', 'lat'=>$key['lat'], 'lng'=>$key['lng'] );
}
/* Convert data to json */
$markers = json_encode( $locations );

Em seguida, defina os marcadores do mapa do google usando os marcadores de variável php.

<script type='text/javascript'>
        <?php
            echo "var markers=$markers;\n";

        ?>
     function initMap() {

            var latlng = new google.maps.LatLng(-33.92, 151.25); // default location
            var myOptions = {
                zoom: 10,
                center: latlng,
                mapTypeId: google.maps.MapTypeId.ROADMAP,
                mapTypeControl: false
            };

            var map = new google.maps.Map(document.getElementById('map'),myOptions);
            var infowindow = new google.maps.InfoWindow(), marker, lat, lng;
            var json=JSON.parse( markers );

            for( var o in json ){

                lat = json[ o ].lat;
                lng=json[ o ].lng;
                name=json[ o ].name;

                marker = new google.maps.Marker({
                    position: new google.maps.LatLng(lat,lng),
                    name:name,
                    map: map
                }); 
                google.maps.event.addListener( marker, 'click', function(e){
                    infowindow.setContent( this.name );
                    infowindow.open( map, this );
                }.bind( marker ) );
            }
        }

O código geral será:

 require_once "db/db_handle.php";
    $select = "SELECT * FROM map";
    $data = $db->query($select);
    foreach ($data as $key)
                $locations[]=array( 'name'=>'Location Name', 'lat'=>$key['lat'], 'lng'=>$key['lng'] );
    }
    /* Convert data to json */
    $markers = json_encode( $locations );
    ?>
    <!DOCTYPE html>
    <html>
      <head>
        <style>
          #map {
            height: 400px;
            width: 100%;
           }
        </style>
      </head>
      <body>
        <h3>My Google Maps Demo</h3>
        <div id="map"></div>
        <script type='text/javascript'>
        <?php
            echo "var markers=$markers;\n";

        ?>

        function initMap() {

            var latlng = new google.maps.LatLng(-33.92, 151.25); // default location
            var myOptions = {
                zoom: 10,
                center: latlng,
                mapTypeId: google.maps.MapTypeId.ROADMAP,
                mapTypeControl: false
            };

            var map = new google.maps.Map(document.getElementById('map'),myOptions);
            var infowindow = new google.maps.InfoWindow(), marker, lat, lng;
            var json=JSON.parse( markers );

            for( var o in json ){

                lat = json[ o ].lat;
                lng=json[ o ].lng;
                name=json[ o ].name;

                marker = new google.maps.Marker({
                    position: new google.maps.LatLng(lat,lng),
                    name:name,
                    map: map
                }); 
                google.maps.event.addListener( marker, 'click', function(e){
                    infowindow.setContent( this.name );
                    infowindow.open( map, this );
                }.bind( marker ) );
            }
        }
        </script>
        <script async defer
        src="https://maps.googleapis.com/maps/api/js?key=myKey&callback=initMap">
        </script>
      </body>
    </html>