your page will have a line similar to this that sends the api keyL
<script type="text/javascript" src="http://maps.google.com/maps?file=api&v=2&key=ABQIAAAAvZMU4-DFRYtw1UlTj_zc6hT2yXp_ZAY8_ufC3CFXhHIE1NvwkxQcT1h-VA8wQL5JBdsM5JWeJpukvw">/*wagmp*/</script>
change that to:
<?php
switch ((isset($_SERVER["SERVER_NAME"]))?$_SERVER["SERVER_NAME"]:"") {
case "domain1.com":
$key = "key1";
break;
case "domain2.com":
$key = "key2";
break;
case "domain3.com":
$key = "key3";
break;
default:
$key = "key1";
break;
}
?>
<script type="text/javascript" src="http://maps.google.com/maps?file=api&v=2&key=<?php echo $key; ?>">/*wagmp*/</script>