Mysqli not importing letters - php

Question

I am trying to make a sign up PHP for my website and I am trying to convert an old script that used mysql to mysqli. I am having a problem where that when I type any letters (abc) into any of the text fields the data is not imported into the database. If I use numbers (123) in all of the boxs it works and gets imported fine. I have tried mixing it up with some letters for the username and numbers for the password to see if only one text box was causing the problem but ANY box that have a letter in will cause the script not to work.
This is my PHP script:
<?php
$mysqli = new mysqli("localhost","root","","users_db");
if ($mysqli->connect_errno) {
echo "Failed to connect to MySQL: (" . $mysqli->connect_errno . ") " . $mysqli->connect_error;
}
/* Define username */
if(isset($_POST['username'])){
$username = $_POST['username'];
}
/* Define email */
if(isset($_POST['email'])){
$email = $_POST['email'];
}
/* Define password */
if(isset($_POST['password'])){
$password = $_POST['password'];
}
/* Define cpassword */
if(isset($_POST['cpassword'])){
$cpassword = $_POST['cpassword'];
}
if (trim($username) == ''){
echo 'No username entered.';
exit();
}
if (strlen($username) <= 5 || strlen($username) >= 30){
echo 'Username needs to be between 5 and 30 characters';
exit();
}
if (trim($email) == ''){
echo 'No email entered.';
exit();
}
if (trim($password) == ''){
echo 'Invalid password.';
exit();
}
if ($password != $cpassword){
echo 'Passwords do not match';
exit();
}
$run = mysqli_query($mysqli, "SELECT * FROM users WHERE username='$username'");
if (mysqli_num_rows($run)>0){
echo 'Username already exists';
exit();
}
$import = "INSERT INTO users (username,email,password) VALUES ($username,$email,$password)";
if (mysqli_query($mysqli, $import)){
echo 'Registration Successful';
$result = mysqli_query($mysqli, "SELECT * FROM users WHERE username='$username'");
$row = mysqli_fetch_array($result);
$id = $row['id'];
mkdir("../users/" . $id, 0777, true);
fopen("../users/" . $id . "/" . "New User.txt", "w") or die("Unable to create file");
}else{
echo 'Failed to import';
}
?>
I am very new to PHP and mysqli so don't be too harsh if I am doing something stupid :)

Answer 1

Thanks Fred -ii- putting quotes around ($username,$email,$password) worked for me and now everything works. I will also fix the other problems suggested above.