city = "'s-Hertogenbosch";

/* this query with escaped $city will work */
$query = sprintf("SELECT CountryCode FROM City WHERE name='%s'",
    mysqli_real_escape_string($mysqli, $city));
$resu = mysqli_query($mysqli, $query);
printf("Select returned %d rows.\n", mysqli_num_rows($result));

/* this query will fail, because we didn't escape $city */
$query = sprintf("SELECT CountryCode FROM City WHERE name='%s'", $city);
$result = mysqli_query($mysqli, $query);