php - How do I fix and call this function every second? -

i have pasted code below can't seem work. i'd achieve when types in username box, i'd echo below, i'd function check field every second. thank you!

name:  <input type="text" name="username" /><br />  <?php $username = $_get['username'];  function functionname() { echo '<p>id: ',$username, "</p>"; } functionname(); ?> 

php isn't right language that, want in javascript achieve this. have @ question detecting input change in jquery?