Running php functions on the same screen - php

Question

Hi I'm very very new to php and playing with codes to learn. I have a very basic login form that has username and password input and two input buttons
one to create entries in the database and other to read from database.
JsFiddle: https://jsfiddle.net/e93kcpto/
My sign up and read functions are located in functions.php file
function signUpFunct(){
if(isset($_POST['username']) and isset($_POST['password'])){
$username = $_POST['username'];
$password = $_POST['password'];
$connection = mysqli_connect('localhost','root','','loginPage');
if($connection){
$query = "INSERT INTO users(username,password) VALUES ('$username','$password')";
if($query){
mysqli_query($connection,$query);
}
else{
die("Sign Up Failed");
}
}
else{
die("Failed to Connect Database");
}
}
}
function readAll(){
$connection = mysqli_connect('localhost','root','','loginPage');
if($connection){
$query = "SELECT * FROM users";
if($query){
$result = mysqli_query($connection,$query);
while($row = mysqli_fetch_assoc($result)){
?>
<pre>
<?php print_r($row);?>
</pre>
<?php
}
}
else{
die("Sign Up Failed");
}
}
else{
die("Failed to Connect Database");
}
}
and this is how I call these two functions in the functions.php file(I'm not sure if this is a good approach so if there is a better way please let me know)
if(isset($_POST['signUp'])){
signUpFunct();
}
else if(isset($_POST['display'])){
readAll();
}
both functions work fine but the problem is when I click on the display button in the browser I am at http://localhost.../functions.php
what I would like to do is to display the data on the form page when I click on the Display button. How can I do this?