You could use JSON.stringify(array) to encode your array in JavaScript, and then use $array=json_decode($_POST['jsondata']); in your PHP script to retrieve it.

AJAX requests are no different from GET and POST requests initiated through a <form> element. Which means you can use $_GET and $_POST to retrieve the data.

When you're making an AJAX request (jQuery example):

// JavaScript file

elements = [1, 2, 9, 15].join(',')
$.post('/test.php', {elements: elements})

It's (almost) equivalent to posting this form:

<form action="/test.php" method="post">
  <input type="text" name="elements" value="1,2,9,15">
</form>

In both cases, on the server side you can read the data from the $_POST variable:

// test.php file

$elements = $_POST['elements'];
$elements = explode(',', $elements);

For the sake of simplicity I'm joining the elements with comma here. JSON serialization is a more universal solution, though.

Here's a function to convert js array or object into a php-compatible array to be sent as http get request parameter:

function obj2url(prefix, obj) {
        var args=new Array();
        if(typeof(obj) == 'object'){
            for(var i in obj)
                args[args.length]=any2url(prefix+'['+encodeURIComponent(i)+']', obj[i]);
        }
        else
            args[args.length]=prefix+'='+encodeURIComponent(obj);
        return args.join('&');
    }

prefix is a parameter name.

EDIT:

var a = {
    one: two,
    three: four
};

alert('/script.php?'+obj2url('a', a)); 

Will produce

/script.php?a[one]=two&a[three]=four

which will allow you to use $_GET['a'] as an array in script.php. You will need to figure your way into your favorite ajax engine on supplying the url to call script.php from js.