mysqli_query() expects parameter 1 to be mysqli, null given in

mysqli_query() expects parameter 1 to be mysqli, null given in

My error messages
Warning: mysqli_query() expects parameter 1 to be mysqli, null given in
/home/orikina/public_html/peacce/php/peacce.sys.php on line 30
Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, null
given in /home/orikina/public_html/peacce/php/peacce.sys.php on line 31
My code:
$usernameCheck = mysqli_query($myCon, "SELECT * FROM `users` WHERE
`username` = 'username'");
if (mysqli_num_rows($usernameCheck) > 0)
{
mysqli_query($myCon, "INSERT INTO users (username,
password, email)
VALUES ('$username', '$passwordReal', '$email')")
or die();
$usernameCheck = mysqli_query($myCon, "SELECT * FROM
users WHERE `username` = 'username'");
if (mysqli_num_rows($usernameCheck) > 0)
{
header("location: ../register.php?data=registerSuccess");
}
else
{
header("location:
../register.php?data=registerError&&refine=3");
}
}