Sql查询(select * where)在php中不起作用

    <?php
$mysqli_host = 'localhost';
$mysqli_user = 'root';
$mysqli_pass = '';
$mysqli_db = 'fives';
$link = mysqli_connect($mysqli_host,$mysqli_user,$mysqli_pass,$mysqli_db);
if (isset($_GET['username'])) {
    $username = mysqli_real_escape_string($link,$_GET['username']);
    print $username. "<br>";
    if (!empty($username)) {
        $username_query = mysqli_query($link,"SELECT * FROM users");
            while($row = mysqli_fetch_array($username_query)) {
                print "username: " . $row["username"]. " - password: " . $row["Password"]. " - date:" . $row["date"]. "<br>";
            }
            print $username_result = mysqli_num_rows($username_query) . "<br>";//print 1
            $username_query = mysqli_query($link,"SELECT * FROM users WHERE username = '" . $username . "'");
            while($row = mysqli_fetch_array($username_query)) {
                print "username: " . $row["username"]. " - password: " . $row["Password"]. " - date:" . $row["date"]. "<br>";
            }
            print $username_result = mysqli_num_rows($username_query);//print 0
    }
}
?>

输出的是

yariv
username: "yariv" - password: "1234" - date:2015-08-26
1
0

嗯，第一个查询没有问题。它得到所有的数据从[用户]这是1行，但当我试图通过使用where username = '" . $username . "' sql查询得到0行。为什么会这样?