How validate forms without js jq - php

Question

Hello i do have a problem,i do not use js or jq just bootstrap link in head,it shoud pop up if forms are invalid but for some reason it wont,thats my prob.On some my other codes it is working but now it wont here i dont know why,i wouldnt post this,but struggling with trivials like this for a day :( dont give me minuses,i dont ask for plus but neither - I would appreciate hlp!
*Signup back*
<?php
if(isset($_POST['signup-submit'])){
require 'db.inc.php';
$error = false;
$username = $_POST['uid'];
$email = $_POST['mail'];
$pwd = $_POST['pwd'];
$pwdrpt = $_POST['pwd_repeat'];
if (empty($username) || empty($email) || empty($pwd) || empty($pwdrpt)) {
header("location: ../signup.php?error=emptyfields&uid=".$username."&mail=".$email);
exit();
}
// else if (!filter_var($email, FILTER_VALIDATE_EMAIL) && !preg_match("/^[a-zA-Z0-9]*$/", $username)){
// header("location: ../signup.php?error=invalidmail&uid");
// exit();
// }
elseif (!filter_var($email, FILTER_VALIDATE_EMAIL)){
$error = true;
$errorEmail = 'Please input a valid Email address';
header("location: ../signup.php?error=invalidmail&uid=".$username);
exit();
}
elseif (!preg_match("/^[a-zA-Z0-9]*$/", $username)){
$error = true;
$errorUsername = 'Please input username';
header("location: ../signup.php?error=invaliduid&mail=".$email);
exit();
}
*SignUp front*
<form action="includes/signup.inc.php" method="post" novalidate>
<h3 align="center"><strong>Sign Up<strong></h3>
<!-- <?php
if (isset($_GET['error'])) {
if ($_GET['error'] == 'invalidmail') {
echo '<p class="signuperror">Enter valid email address!</p>';
}
}
?> -->
<input class="form-control" type="text" placeholder="Username" name="uid">
<div><span class="text-danger"><?php if(isset($errorUsername)) echo $errorUsername; ?></span>
</div>
<br>
<input class="form-control" type="text" placeholder="Enter Email" name="mail">
<div><span class="text-danger"><?php if(isset($errorEmail)) echo $errorEmail; ?></span></div>

Answer 1

Try this
<?php
if(isset($_POST['signup-submit'])){
$error = false;
$username = $_POST['uid'];
$email = $_POST['mail'];
if (empty($username) || empty($email)) {
header("location: index1.php?error=emptyfields");
exit();
}
elseif (!filter_var($email, FILTER_VALIDATE_EMAIL)){
$error = true;
$errorEmail = 'Please input a valid Email address';
header("location: index1.php?error=invalidmail");
exit();
}
elseif (!preg_match("/^[a-zA-Z0-9]*$/", $username)){
$error = true;
$errorUsername = 'Please input username';
header("location: index1.php?error=invaliduid");
exit();
}
}
?>
<form action="" method="post" novalidate>
<h3 align="center"><strong>Sign Up<strong></h3>
<?php
if (isset($_GET['error'])) {
if ($_GET['error'] == 'invalidmail') {
echo '<p class="signuperror">Enter valid email address!</p>';
}
}
?>
<input class="form-control" type="text" placeholder="Username" name="uid">
<div><span class="text-danger"><?php if(isset($errorUsername)) echo $errorUsername; ?></span>
</div>
<br>
<input class="form-control" type="text" placeholder="Enter Email" name="mail">
<div><span class="text-danger"><?php if(isset($errorEmail)) echo $errorEmail; ?></span></div>
<input type="submit" name="signup-submit" id="signup-submit">
</form>