Without reloading, using HTML and PHP only it is not possible, but this can be very similar to what you want, but you have to reload:
<?php
function test() {
echo $_POST["user"];
}
if (isset($_POST[])) { // If it is the first time, it does nothing
test();
}
?>
<form action="test.php" method="post">
<input type="text" name="user" placeholder="enter a text" />
<input type="submit" value="submit" onclick="test()" />
</form>