Dynamic Banner Text based on Subdomain

You can put you code in functions.php where you define the string, for example:

<?php   
function get_banner_text() {
  $url = $_SERVER["REQUEST_URI"];
  if (strpos($url, "bob."))  {
    $banner_text = "Welcome to Agent Bob's Portal!";
  } else {
    $banner_text = "Default text here";
  }
  return $banner_text;
}
?>

Afterwards for example in header.php of your theme you could have like:

<div class="banner"><?php echo get_banner_text(); ?></div>