菜鸟送分题！

a.php:<?php
    if ( isset( $_GET['value'] ) )
        echo $_GET['value'];
?>
<html>
<head><title>a.php</title>
</head>
<body>
<a href="a.php?value=1">1</a><br>
<a href="a.php?value=2">2</a><br>
<a href="a.php?value=3">3</a><br>
<a href="a.php?value=4">4</a>
</body>
</html>