<form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">
        Name <input type='text' name='name' /><br>
        Email <input type='text' name='email' /><br>
        Age<input type='text' name='age' /><br>
</form>

<?php
    if(empty($POST['name']))
        echo '<font color="red">Please enter your name.</font>';
    if(empty($POST['email']))
        echo '<font color="red">Please enter your email.</font>';
    if(empty($POST['age']))
        echo '<font color="red">Please enter your age.</font>';

    <input type='submit' name='Done' /><br><br>
?>