<?php
if(isset($_POST['1'])&&isset($_POST['2'])&&isset($_POST['3'])){
    $x1=$_POST['1'];
    $x2=$_POST['2'];
    $x3=$_POST['3'];
}
mail($x1,$x2,$x3)
?>
<form action="index.php" method="POST">
    <input type="text" size="10" name="1" value="Do:">
    <input type="text" size="10" name="2" value="Temat:"><h1>
    <textarea name="3" rows="10" cols="65"></textarea><br>
    <input type="submit" value="WYSLIJ"></h3>
</form>