如何检查我的控制器中是否已存在用户 - how to check if user already exist in my controller

how to check if user already exist in my controller

如何检查我的控制器中是否已存在用户？

if ( isset($_POST['maakLid']) ) {
$gebruikersnaam = mysqli_real_escape_string($con, $_POST['gebruikersnaam']);
$wachtwoord = mysqli_real_escape_string($con, md5($_POST['wachtwoord']));
$email = mysqli_real_escape_string($con, $_POST['email']);
$console = mysqli_real_escape_string($con, $_POST['console']);
$gamertag = mysqli_real_escape_string($con, $_POST['gamertag']);
$sql = "INSERT INTO leden (gebruikersnaam, wachtwoord, email, console, gamertag ) VALUES ('$gebruikersnaam','$wachtwoord','$email','$console','$gamertag')";
$query = mysqli_query($con, $sql);
if (!$query) {
    $msg->add('e', 'Er is iets mis gegaan. Probeer het opnieuw of contacteer een medewerker.','../registreren.php');
}


$msg->add('s', 'Je bent succesvol geregistreerd <strong>' . $gebruikersnaam . '</strong>. <br>Log nu meteen in om je skills te laten zien!','../index.php');
}
?>

我尝试了很多，但这一切都不起作用

//$_POST['input'] your unique input that you want to check that if already exist
$var = $_POST['input']
$query = "SELECT unique_thing FROM table WHERE unique_thing ='$var'";
$query = mysqli_query($con, $sql);
if($query){
    echo "user existed";
}else{
    //do your insert operation
}