what does header function do in php
<?php
// PHP program to describes header function
// Set a past date
header("Expires: Sun, 25 Jul 1997 06:02:34 GMT");
header("Cache-Control: no-cache");
header("Pragma: no-cache");
?>
<html>
<body>
<p>Hello World!</p>
<!-- PHP program to display
header list -->
<?php
print_r(headers_list());
?>
</body>
</html>