PHP header auto refresh doesn't always work

I have a login script where I do some integrity testing for the user submission. If all is fine, I set a session variable and do a refresh.  The code looks like this:

       $url = "https://mysite.com/index.php";
	header('refresh: 0; url='.$url); 
	exit; // to keep the rest of the form from continuing

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What happens every once and a while (like 1 in 100) is that the auto refresh doesn't fire and since there isn't any text on the page it appears as if the page has frozen.

I just added this line which at least puts something up on the person's page. Otherwise all they got was a "white screen of death"...
      print 'Your page should auto refresh in two seconds. If not, please click this link: <a href="'.$url.'">'.$ur2.'</a>';

So now people at least see something other than the white screen.

Any insight on how to fix this so that it fires for everybody?