Not with PHP it isn't.
You could probably use JS for that, but then I'd have to beg the question as to why you'd bother with using a form instead of choosing a hyperlink?
You don't need two scripts. You can use a self posting one.
I would use one script and expand it.
Or something like that.
$aValidLinks = array(
'Site Display Name' => 'siteurl',
'Site Display Name 2' => 'siteurl 2',
//. . .
if (in_array($_GET['selSite'], $aValidLinks))
header('Location: ' . $_GET['selSite']);
printf("%s is an invalid selection." . PHP_EOL, htmlspecialchars($_GET['selSite']));
<!-- . . . -->
<form method="get" action="<?php echo $_SERVER['SCRIPT_NAME'];?>">
<optgroup label="Select Site">
foreach ($aValidLinks AS $title => $url)
printf(' <option value="%s">%s</option>' . PHP_EOL, $url, $title);