is an outbound POST possible from inside a jQuery function?

is an outbound POST possible from inside a jQuery function?

I have a specific problem in passing a string from Autocomplete to a php script, but in doing a lot of research, I'm getting a hint that the problem may be general with jQuery functions.  The code below does not work, in that php prints 'country is' but not the string variable that is received via POST. After hours of searching, I found an Autocomplete documentation section that says its about plugins, but its not, its really about further options and features of Autocomplete that are not mentioned in the Autocomplete Documentation and Demo section. This information is at http://docs.jquery.com/Plugins/autocomplete   This page is not linked to or referenced by anything in the Docs and Demo section.  The 'plugins' section mentions an Autocomplete option extraParams:  There is no discussion or example of how to use extraParams:  but the context suggests that may be the way to pass a string out.  It doesn't say so, but I'm wondering: 1. is that because POST won't work from inside a jQuery function, and 2. how and when should extraParams be used? (a demo would be really nice)  Anyway, here's the code that does not work. What needs to be done to get the php to print 'country is US"

jQuery

        $("#stateProvince").autocomplete
        ({
            source: function( request, response )
            {            
                countryFilter='countryAbbrev=' + $('input[name=country]:checked').val();   //eg. countryAbbrev=US
                alert('|' + countryFilter + '|2nd');
                $.ajax(
                {
                    url: "getStateProvince.php",                   
                    type: "POST",   // a jQuery ajax POST always transmits in utf-8
                    dataType: "json",   //expect returned data to be in json format                   
                    data: countryFilter,                   
                    success: function( return_arr )   //more is needed here, but I'd like to get a string passed first

php

<?php
$country=$_POST['countryAbbrev'];
print "country is {$country} <br>";
?>