Php server side validation without using $_SERVER["PHP_SELF"]

Question

I am fairly new to server-side validation and I currently post my form values to the page below.

<form id="sign_in" action="<?php echo htmlspecialchars('inc/validate.php') ?>" method="POST">

I perform other operations on the page so I'd like all the code to be in one place. I would like to handle all of my validation on this page and send back the error messages to the form page. However, alot of the examples I'm seeing are using:

action="<?php echo $_SERVER['PHP_SELF']; ?>"

validate.php has the following code

<?php
include('prc_input_validation.php');


// check if username and password isset and are not empty
if ($_SERVER['REQUEST_METHOD'] == 'POST') {



   // Retrieve values from login form

    $username = checkInput($_POST['username']);
    $password = checkInput($_POST['password']);
    
    $userNameempty = emptyCheck($username);
    if($userNameempty == "true")
    {
        $empty_error = "This field is required";
        header("location: ../sign-in.php");
    }

}

?>

prc_input_validation.php has the following code

<?php

function checkInput($inputField) {
    $inputField = trim($inputField); // Strip whitespace (or other characters) from the beginning and end of a string
    $inputField = stripslashes($inputField);  //Un-quotes a quoted string
    $inputField = htmlspecialchars($inputField); //Convert special characters to HTML entities
    return $inputField;
}  


    function emptyCheck($inputField)
    {
        if(empty($inputField))
        {
            return "true";
        }
        else
        {
            return "false";
        }
    }
    
    ?>

Form input

           <input type="email" class="form-control" name="username" placeholder="Username" autofocus required>
                        <span class="text-danger"><?php echo $empty_error; ?></span>  

How can I correctly send back the error message to the form for it to be displayed? on the form