Dynamicaly change page title in php

<?php ob_start(); //top of the page.
?>
<title>%TITLE%</title>

<?php 
echo page_title("page title");

function page_title($title) {  //dinamicaly set page title

$buffer=ob_get_contents();
ob_clean();
$buffer=str_replace("%TITLE%",$title,$buffer);
return $buffer;

}
?>