<form action="validate.php" method="post">
    <p>Username: </p><input type="text" name="username" value="" /><br />
    <p>Password: </p><input type="password" name="password" value="" /><br />
    <br />
    <input type="submit" name="submit" value="submit" /><br />
</form>

<pre>
<?php
    print_r($_POST);
?>
</pre>

<?php
    // Uses ternary operator
    $username = isset($_POST["username"]) ? $_POST["username"] : NULL;
    $password = isset($_POST["password"]) ? $_POST["password"] : NULL;
?>

if (isset($_POST["username])) {
    $username = $_POST["username"];
} else {
    $username = "";
}

if (isset($_POST["password"])) {
    $password = $_POST["password"];
} else {
    $password = "";
}