how to get GET and POST variables with JQuery?

For GET parameters, you can grab them from document.location.search:

var $_GET = {};

document.location.search.replace(/\??(?:([^=]+)=([^&]*)&?)/g, function () {
    function decode(s) {
        return decodeURIComponent(s.split("+").join(" "));
    }

    $_GET[decode(arguments[1])] = decode(arguments[2]);
});

document.write($_GET["test"]);

For POST parameters, you can serialize the $_POST object in JSON format into a <script> tag:

<script type="text/javascript">
var $_POST = <?php echo json_encode($_POST); ?>;

document.write($_POST["test"]);
</script>

While you’re at it (doing things on server side), you might collect the GET parameters on PHP as well:

var $_GET = <?php echo json_encode($_GET); ?>;

Note: You’ll need PHP version 5 or higher to use the built-in json_encode function.

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Update: Here’s a more generic implementation:

function getQueryParams(qs) {
    qs = qs.split("+").join(" ");
    var params = {},
        tokens,
        re = /[?&]?([^=]+)=([^&]*)/g;

    while (tokens = re.exec(qs)) {
        params[decodeURIComponent(tokens[1])]
            = decodeURIComponent(tokens[2]);
    }

    return params;
}

var $_GET = getQueryParams(document.location.search);