PHP表单复选框和未定义的索引

<form method="post" action="">
   <label<input type="checkbox" name="favorite_colors[]" value="red"> Red</label>
   <label<input type="checkbox" name="favorite_colors[]" value="green"> Green</label>
   <label<input type="checkbox" name="favorite_colors[]" value="blue"> Blue</label>
   <input type="submit" name="submit" value="Submit">
</form>

<?php
if (isset($_POST['submit'])) {
    $favorite_colors = isset($_POST['favorite_colors']) ? $_POST['favorite_colors'] : array();
    foreach ($favorite_colors as $color) {
        echo "Favorite color: $color<br>";
    }
}
?>