Changing header image by time?

Hi
I would like to change the header image by time.
I can customize that in custom-header.php, but i can’t get the code running.

The code i would insert is the following, but i can’t get it working…

<?php
$hour = date('G');
if($hour > 19) {
$image = "/images/header_nacht.jpg";
$alt = "Mornin' all.";
}
if($hour > 6) {
$image = "/images/header_tag.jpg";
$alt = "Where does the day go.";
}

echo "<img src="$image" alt="$alt" />";
?>