How to validate two forms at once with php - php

Question

I have two forms contained in one PHP file. One of the forms is for "log in" and the other form is to "sign up" i have some CSS and JS that will switch out the forms visually, but regardless the two forms are still in one php file.
These forms are standard:
<form action="login.php" method="post" name="user-sign-up">
<div class="error">
<?php if(!empty($error)){echo $error;} ?>
</div>
<span>First name</span>
<input type="text" name="fname" class="fname">
<input type="submit" value="Sign Up" class="signupdawg" id="firstname-signup">
</form>
Then I got another form underneath that one.
Here's my php, it sort of works and only works for one of the forms:
if( isset($_POST['fname']) && !empty($_POST['fname'])){
$upName = $_POST['fname'];
if(empty($upName)){
$error = 'All fields are required';
I only shortened the code, so it wouldn't take up to much space. I had other inputs like a password, but I just removed it because the code is pretty much the same except I had an "or" in my php empty check and a few more lines in my html for it.
Anyway, I am wondering how I can validate two forms at once with PHP.
Thank you.

Answer 1

You can only submit one form at a time, so you need to use something on the form to determine which one was submitted. This is just a simple example using your code:
if(isset($_POST['Sign_Up'])) {
//do sign up stuff
}
elseif(isset($_POST['Login'])) {
//do login stuff
}
Notice Sign Up is converted to Sign_Up. It may be better to use two separate action in each form and have two different files.
Additionally, isset is redundant here:
if( isset($_POST['fname']) && !empty($_POST['fname'])){
The empty already checks if it is set, so just:
if(!empty($_POST['fname'])){

Answer 2

You need to use a unique value on the submit button for each form, an example is below
index.html
<form method="post" ...>
...
<button name="submit" type="submit" value="login">login</button>
</form>
<form method="post" ...>
...
<button name="submit" type="submit" value="sginup">sginup</button>
</form>
PHP file
<?php
if (isset($_POST["submit"])) {
switch ($_POST["submit"]) {
case "login":
login();
break;
case "sginup":
singup();
break;
default:
break;
}
}
?>