Making a xhtml valid searchform using a php if/else statement

I’m trying to make a xhtml valid search form using an if/else php statement. I want the searchform to say ‘Search…’ before you type into it, but as soon as you type, it needs to show what you are typing.

This is the code I have so far:

value=" <?php
if
  echo "Search";
else
  echo "<?php the_search_query(); ?>";
?>

I want the if statement to be something along the lines of if the search query is blank, however I have no idea how to write this in php.

Any help will be greatly appreciated!