how to echo in a specific place

i just want to know how to echo in a specific place if the code was outside of body and html

thx Eric

<?
$var="not much";
?>

<body>
<h1><?="Hello world"?></h1><br>
<h2><?php echo "whatsup"?></h2><br>
<h3><?=$var?></h3><br>

that kinda thing?