How can I do a php redirect?

Hello,
I am trying to do a simple php redirect by putting the following code at the very top of my header.php file. But I get a warning saying that output has been started at line 2. Which is the line that says <?php. I don’t understand why this is happening. I am not echoing anything. Is WP loop causing this? should I make changes? Please help.

<?php if(have_posts()) : while(have_posts()) : the_post();?>
 <?php
  $externalLink = get_post_meta($post->ID,'link',TRUE);
  if($externalLink) {
   header("Location: $externalLink");
  }
 ?>
<?php endwhile; endif;?>