qui http://www.html.it/articoli/google-maps-v3-geocoding/ trovi una guida che te lo spiega, comunque devi fare così, crei una pagina chiamata per esempio mappa.php con il seguente codice
Codice PHP:



<html>
<head>
<meta name="viewport" content="initial-scale=1.0, user-scalable=no"/>
<meta http-equiv="content-type" content="text/html; charset=UTF-8"/>
<title>Google Maps API v3: geocoding</title>
<style type="text/css">
html, body { margin:0; padding:0; width:100%; height:100%; }
body { background:#FFFFFF; color:#000000; font-family:Arial, Helvetica, sans-serif; font-size:12px; line-height:150%; text-align:center;}
#map { width:100%; height:95%; }
input { width:250px; }
#tooltip { padding:10px; text-align:left; }
#tooltip p { padding:0; margin:0 0 5px 0; }
</style>
<script type="text/javascript" src="http://maps.google.com/maps/api/js?sensor=true"></script>
<script type="text/javascript">

var createMap = function() {
    
    searchAddress();

       var address = document.getElementById("address").value;
    
    var geocoder = new google.maps.Geocoder();
    
    geocoder.geocode( {'address': address}, function(results,status) {
        if (status == google.maps.GeocoderStatus.OK) {
        
            var options = {
                  zoom: 12,
                  center: results[0].geometry.location,
                  mapTypeId: google.maps.MapTypeId.ROADMAP
            };    

            var map = new google.maps.Map(document.getElementById('map'), options);
    
            var marker = new google.maps.Marker(
                {
                  position: results[0].geometry.location,
                  map: map,
                  title: 'Spoleto'
                }
            );
    
            var tooltip = '<div id="tooltip">'+
                '

formatted_address:
'+                
                   results[0].formatted_address+'</p>'+
                '

latLng:
'+
                results[0].geometry.location+'</p>'+            
                '</div>';
    
            var infowindow = new google.maps.InfoWindow({
                content: tooltip
            });
    
            google.maps.event.addListener(marker, 'click', function() {
                  infowindow.open(map,marker);
            });
            
          
        } else {
          alert("Problema nella ricerca dell'indirizzo: " + status);
        }
      });
    
}

var searchAddress = function(){
    document.getElementById("submit").onclick = function() {
        createMap();
    }
}

window.onload = createMap;
</script>
</head>
<body>
<div>
    <input id="address" type="hidden" value="<?php echo $_GET['city']; ?>"/>
    <input id="submit" type="hidden" value="trova indirizzo sulla mappa"/>
</div>
<div id="map"></div>
</body>
</html>
se vuoi cercare la città Torino ad esempio usi questo link http://example.com/mappa.php?city=torino