If your _header.php partial file contains HTML output it should be included in the body:
<!doctype html>
<html>
<link href="stylesheets/main_stylesheet.css" rel="stylesheet" type="text/css">
<link href='http://fonts.googleapis.com/css?family=Slabo+27px' rel='stylesheet' type='text/css'>
<head>
<meta charset="utf-8">
<title>Welcome</title>
</head>
<body>
<?php include("_header.php"); ?>
This is the body of this page
</body>
</html>