jQuery UI自动完成多个远程（JSON，PHP，JS）

I want to make a jQuery UI Autocomplete with multiple values and remote (https://jqueryui.com/autocomplete/#multiple-remote).

Now I have these codes:

JS

$(function() {
    function split( val ) {
      return val.split( /,\s*/ );
    }
    function extractLast( term ) {
      return split( term ).pop();
    }

    $("#retr")
        .bind( "keydown", function( event ) {
            if ( event.keyCode === $.ui.keyCode.TAB &&
                $( this ).autocomplete( "instance" ).menu.active ) {
                event.preventDefault();
            }
        })
        .autocomplete({
            source: function( request, response ) {
                $.getJSON( "retr/retr_arry.php", {
                term: extractLast( request.term )
            }, response );
        },
        search: function() {
            var term = extractLast( this.value );
            if ( term.length < 3 ) {
                return false;
            }
        },
        focus: function() {
            return false;
        },
        select: function( event, ui ) {
            var terms = split( this.value );
            terms.pop();
            terms.push( ui.item.value );
            terms.push( "" );
            this.value = terms.join( ", " );
            return false;
        }
    });
});

HTML

<input id="retr" type="text" placeholder="Please select the user(s)">

PHP

//Database connect...

if (isset($_POST["term"])) {
    $input_term = $_POST["term"];
} else {
    exit; 
}
$username = array();
$term2 = '%'.$term.'%';

//SQL
$sql = $db->prepare("SELECT username FROM users WHERE username LIKE ?");
$sql->bind_param('s', $term2);
if ($sql->execute()) {
    $sql->store_result();
    $sql->bind_result($username);
    while ($sql->fetch()) {
        $usernames[] = $username;
    }
    echo (json_encode($usernames));
} else {
    exit;
}

The Problem

I get no autocomplete. When I enter up to 5 letters I still get no results.

Edit

After changing POST to GET (in the PHP file), I get results. But the results aren't sorted correctly.

For example: If I enter "adm" I get "SYSTEM". How can I fix this? I think the problem is in the SQL part of the PHP.

if (array_key_exists("term", $_POST)) {
    $term2 = '%'.$_POST["term"].'%';
} else {
    exit;
}
$username = array();

//SQL
$sql = $db->prepare("SELECT username FROM users WHERE username LIKE ?");
$sql->bind_param('s', $term2);
if ($sql->execute()) {
    $sql->store_result();
    $sql->bind_result($username);
    while ($sql->fetch()) {
        $usernames[] = $username;
    }
    echo json_encode($usernames);
} else {
    exit;
}

instead of use exit... better return an error message / return the proper HTTP State, so you can handle this cases instead of dropping them and crash your javascript.

Update: An hint: instead of using mysqli use PDO. PDO is forward-looking and supports many database driver. so it's easy to switch them and so on. PDO is easier and your code could be smaler and better to understand.