E' un problema di sintassi Javascript, non di PHP:
Codice PHP:
<script type="text/javascript">
<!--
var array = new Array();
array["pippo"] = "Hello World!";
alert(array["pippo"]);
array[pluto] = "Hello World!"; // Warning 'pluto' is undefined
alert(array[pluto]); // Warning 'pluto' is undefined
-->
</script>