PHP: give alert popup then redirect the page

I am new to PHP.

When someone uploads a file size too big, I want to show them a warning popup and redirect them to a previous page (or vice versa).

if(file size is too big){    
   ob_start();   
   header("location:index.php");    
   echo "<script type='text/javascript'>alert('Your File Size is too big!');</script>";   
   ob_end_flush();   
   exit;    
}

This code above will just redirect me to index.php and doesn't show any warning popup.