Can't pass a variable from one php file to another - php

Question

I have a simple login page, and the idea is that if user input incorrect passowrd/login, then he or she will see error message on the same page. I have code in 3 different files - one is simple html, another has the functions, and last one runs all the logic:
<div id="content">
<div class="logo-container"><img src="images/logo2.png" alt=""></div>
<div class="container-fluid">
<!-- All login logic is in login.php file -->
<form action='/login-logic.php' method="post" class="form-1">
<p>
<label for="username" class="sr-only">Username</label>
<input type="text" class="form-control" id="username"
name="username" placeholder="What's your username?" required />
</p>
<p>
<label for="password" class="sr-only">Password</label>
<input type="password" class="form-control" id="password"
name="password" placeholder="What's your password?" required/>
<?php
if($isValid === false) {
echo "<div id='alert-message' class='alert alert-danger'>SCRUB</div>";
}
?>
</p>
<p class="submit">
<button id="submit-button" type="submit" name="submit" value="submit"><i class="fa fa-arrow-right"></i></button>
</p>
</form>
</div>
// Check match of the credentials in database
function loginValidation($query) {
global $isValid;
$isValid = true;
$count = mysqli_num_rows($query);
if($count == 1) {
header('Location: pages/signup.php'); /* Redirect browser */
} else {
$isValid = false;
header('Location: index.php');
/* Redirect browser */
}
}
Thank you!

Answer 1

You declare a variable just before to force browser to reload the page. So the variable is no more defined in the next request.
Here is a possible way.
Instead of :
{
$isValid = false;
header('Location: index.php');
/* Redirect browser */
}
Do :
{
/* Redirect browser */
header('Location: index.php?error');
exit();
}
Then, in HTML :
if (isset($_GET['error'])) {
echo "<div id='alert-message' class='alert alert-danger'>SCRUB</div>";
}