if (!empty($_POST["mail"])) {
echo "Yes, mail is set";
} else {
echo "No, mail is not set";
}<?php
if (isset($_POST['name'])) $name = $_POST['name'];
else $name = '(enter your name)';
echo <<<_END
<html>
<head>
<title>Test</title>
</head>
<body>
Your name is $name<br />
<form method = 'post' action = 'count.php'>
What's your name?
<input type='text' name='name' />
<input type='submit' />
</form>
</body>
</html>
_END
?>
<?php
$a = "3.1416";
if (isset($a)) // la variable $a está definida
unset($a); //ahora ya no está definida
?>