How to optimize mysql query registration - php

Question

At the time of registration, I am checking for there field username, email, phonenumber
With three query. snippet is following:
$query = "SELECT *
FROM
users
WHERE username='$userName'";
$result = mysql_query($query, $this->con);
$count_username = mysql_num_rows($result);
if($count_username <= 0){
$query = "";$result = "";
$query = "SELECT *
FROM
users
WHERE email='$email' ";
$result = mysql_query($query, $this->con);
$count_email = mysql_num_rows($result);
}
if($count_username <= 0 && $count_email <= 0){
$query = "";$result = "";
$query = "SELECT *
FROM
users
WHERE phone_number='$phone'";
$result = mysql_query($query, $this->con);
$count_phone = mysql_num_rows($result);
}
Is there is way to do this with single query, or is there is other way to optimize this code???
Sorry for using mysql extension.
If i use single query:
$query = "SELECT * FROM users WHERE username='$userName' && email='$email' && phone_number='$phone'";
I am unable to show different error:
username exists
email exists
phone exists
I don't want to show error like:
username/phone/exists exists

Answer 1

You can have something like this:
SELECT CASE WHEN username = '$userName' THEN 'username exists'
WHEN email = '$email' THEN 'email exists'
ELSE 'phone exists'
END Result
FROM users
WHERE username = '$userName'
OR email = '$email'
OR phone_number = '$phone'