How to validate forms - php

Question

I am new in PHP programming and I am creating my first registration and login form. I did everything but there is one problem: How I must sort the code. I tried in lots of ways but it is messed up.
if (filter_input_array(INPUT_POST)) {
if (preg_match('/\s/', $name)) {
$errorName = "Names doesn't contain whitespaces";
$mainError = true;
}
$sql = "SELECT name FROM register WHERE name='$name'";
$result = mysqli_query($conn, $sql);
$row = mysqli_fetch_array($result, MYSQLI_ASSOC);
if (mysqli_num_rows($result) == 1) {
$errorName = 'This name is already registered';
$mainError = true;
}
if (mb_strlen($name) > 0 AND mb_strlen($name) < 3) {
$errorName = 'Name too short';
$mainError = true;
}
if (mb_strlen($name) > 15) {
$errorName = 'Name too long';
$mainError = true;
}
if ($name == '') {
$errorName = 'This field is REQUIRED';
$mainError = true;
}
if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
$errorEmail = "Invalid email";
$mainError = true;
}
if (preg_match('/\s/', $pass1)) {
$errorName = "Password mustn't contain whitespaces";
$mainError = true;
}
if (mb_strlen($pass1) > 0 AND mb_strlen($pass1) < 6) {
$errorPass1 = 'Password too short';
$mainError = true;
}
if (mb_strlen($pass1) > 20) {
$errorPass1 = 'Password too long';
$mainError = true;
}
if ($pass1 == '') {
$errorPass1 = 'This field is REQUIRED';
$mainError = true;
}
if ($pass1 != $pass2) {
$errorPass2 = "Passwords don't match";
$mainError = true;
}
if ($pass2 == '') {
$errorPass2 = 'This field is REQUIRED';
$mainError = true;
}
if (!$mainError) {
$feedback = 'You registered successfully!';
}
}
I want to know what must be first