It keeps saying signup=empty

This is the script. I can't figure out where the error is. Can you help me?
I also posted the whole site in a zip file below.

<?php
if (isset($_POST['submit'])) {
 
    include_once 'dbh.inc.php';
 
    // Error handlers
    // Check for empty fields
    if (empty($first) || empty($last) || empty($email) || empty($uid) || empty($pwd)) {
        header("Location: ../signup.php?signup=empty");
        exit();
    } else {
            // Check if input characters are valid
            if (!preg_match("/^[a-zA-Z]*$/", $first) || !preg_match("/^[a-zA-Z]*$/", $last)) {
                header("Location: ../signup.php?signup=invalid");
                exit();
            } else {
                //Check if email is valid
                if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
                    header("Location: ../signup.php?signup=email");
                    exit();
                } else {
                    $sql = "SELECT * FROM users WHERE user_uid='$uid'";
                    $result = mysqli_query($conn, $sql);
                    $resultCheck = mysqli_num_rows($result);
                 
                    if ($resultCheck > 0) {
                        header("Location: ../signup.php?signup=usertaken");
                        exit();
                    } else {
                        //Hashing the password
                        $hashedPwd = password_hash($pwd, PASSWORD_DEFAULT);
                        // Insert the user into the database
                        $sql = "INSERT INTO users (user_first, user_last, user_email, user_uid, user_pwd)
                        VALUES (?,?,?,?,?);";
                        mysqli_query($conn, $sql);
                        $stmt=mysqli_stmt_init ($conn);
                        if (!mysqli_stmt_prepare ($stmt, $sql)) {
                            echo "SQL error";
                         
                        } else {mysqli_stmt_bind_param ($stmt, "sssss", $first, $last, $email, $uid, $pwd);
                        mysqli_stmt_execute ($stmt);
                         
                        }
                        header("Location: ../signup.php?signup=success");
                        exit();
                    }
                }
            }
    }
 
} else {
    header("Location: ../signup.php");
    exit();
}

PHP:

 

you were right. thank you very much!