确保用户名和电子邮件地址是唯一的

I am trying to verify whether or not a username and email address exist in my database and have tried to do a if, elseif, else statement to no avail.

I want to first run a check to see if the username is fine - obviously if not, an echo statement will appear. If the username doesn't exist, run an elseif statement to see if the email address is unique - again if not, another echo statement will appear. For the final statement, if all other conditions return false, I want to run the below code so that the user's input is submitted to the database.

I initially tried to declare two variables with a statement to check if the username=$username and email_address=$email_address then check to see if the number of rows returned from a mysqli_query is more than 1 for the username. I entered an elseif statement with the same but for email address but then I had an else statement with the below code in {} brackets.

I have deleted the original code because too many errors were thrown up, and was probably too convoluted and messy when a more elegant way to do what I was exists.

Any help would be appreciated.

if(isset($_POST['submit']))
    {
$sql = "INSERT INTO users (first_name, last_name, username, email_address, password,    gender, city, country, verification_code, verified, sign_up_date) VALUES (
'$first_name',
'$last_name',
'$username',
'$email_address',
'$password',
'$gender',
'$city',
'$country',
'$verification_code',
'1',
'$sign_up_date')";
    $result = mysqli_query($conn,$sql);
    header("Location:confirmation.php");
}   

What you want is an integrity check on the data. You should do this check inside the database. The simplest way is with unique constraints/indexes:

create unique index unq_users_username on users(username);
create unique index unq_users_email on users(email);

If you attempt to insert or update a row so it violates these constraints, then your data modification step will fail with an error.

You need to create an index for them. Use The following command to create the index:

CREATE UNIQUE INDEX index_name ON table_name (column_name)

Check This Link for more info: https://www.w3schools.com/sql/sql_create_index.asp

You could write a function to check your database first for example:

$errors = []; // you can add errors to this array.
if (isset($_POST['submit']))
{
    // first do your validation here against empty values and invalid email

    $sql = "SELECT * from users where email='$email' and username='$username'";
    $result = mysqli_query($conn, $sql);

    if (mysqli_num_rows($result) > 0) {
       $errors[] = "Username and email has been taken.";
    }

    if (!empty($errors))
    {
       // loop through your errors and echo them
    } else {
      // insert your values into the database 
    }
}