[PHP tags][php-tags] are used to let PHP know which part(s) of a file it needs to parse. Because of this we can mix PHP and another language (e.g. HTML). To start the parsing of code in a file you need to add the PHP opening tag <?php and to end the parsing of code in a file you need to add a PHP closing tag ?>. Everything outside of the PHP tags will be left alone by the parser and will just work like normal. A simple example of this using a simple HTML document:
<html>
<head>
<title><?php echo 'This is the page title'; ?></title>
</head>
<body>
<h1><?php echo 'This is the heading of the page'; ?></h1>
<p>This is a paragraph...</p>
</body>
</html>