PHP Different Image Depending On The Day

<?php
$mon = "random/mon.gif";
$tue = "random/tue.gif";
$wed = "random/wed.gif";
$thu = "random/thu.gif";
$fri = "random/fri.gif";
$sat = "random/sat.gif";
$sun = "random/sat.gif";
$t=date('d-m-Y');
$curlday = date("D",strtotime($t));
$sothisgifistheonetoday = '';
switch (strtolower($curlday)) {
    case 'mon';
        $sothisgifistheonetoday = $mon;
        break;
    case 'tue':
        $sothisgifistheonetoday = $tue;
        break;
    case 'wed':
        $sothisgifistheonetoday = $wed;
        break;
    case 'thu';
        $sothisgifistheonetoday = $thu;
        break;
    case 'fri':
        $sothisgifistheonetoday = $fri;
        break;
    case 'sat':
        $sothisgifistheonetoday = $sat;
        break;
    case 'sun':
        $sothisgifistheonetoday = $sun;
        break;
}
?>
<div align="center"><img src="<?php echo $sothisgifistheonetoday; ?>"</div>