Why is the responseText variable returning undefined, and not accessing the PHP as it should?

Question

This Ajax is not retrieving any information from its' PHP file as it should, and when inspecting the Javascript in Firebug I noticed that responseText is returning undefined.

Here is the Javascript:

function getTeams(sport)
{
    var req = new XMLHttpRequest();
    var nextTeam, nextOption;

    // callback function, using "closure"
    req.onreadystatechange = function()
    {
        if( (req.readyState == 4) && (req.status == 200) )
        {
            var result = req.responseText;

            // array teams created here
            var teams = result.split(", ");

            for(var i = 0; i < teams.length; i++)
            {
                nextTeam = teams[i];
                nextOption = new Option(nextTeam);

                /* add the new option to the option list
                 ("null" for IE5, "-1" for all other browsers) */
                try
                {
                    document.getElementById("teamsList").add(nextOption, -1);
                }
                catch(e)
                {
                    document.getElementById("teamsList").add(nextOption, null);
                }
            }
        }
    } // end of closure

    req.open("GET", "favoriteSport.php?sport=" + sport, true);
    req.send(null);
}<br /><br />

Here is the PHP:

<?php
    $sport = $_GET["sport"];

    $teams = array("basketball" => "Atlanta Hawks, Chicago Bulls, New York Knicks",
                    "baseball" => "Atlanta Braves, Chicago Cubs, New York Yankees",
                    "football" => "Atlanta Falcons, Chicago Bears, New York Giants");

    return $teams[$sport];
?><br /><br />

...and here is the relevant html:

    <select onchange = "getTeams(this.value);">
        <option>
            --select a sport--
        </option>
        <option value = "basketball">
            basketball
        </option>
        <option value = "baseball">
            baseball
        </option>
        <option value = "football">
            football
        </option>
    </select>

    <select id = "teamsList"></select>

Answer 1

It looks like your issue is here:

return $teams[$sport];

Try

echo $teams[$sport];