Catchable fatal error: Object of class mysqli_result could not be converted to string in C:\\xampp\\htdocs\\xxx\\dash.php on line 20
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You have to fetch it first before echoing the results. Rough Example:
function GetVar($var, $username, $mysqli) {
// make the query
$query = $mysqli->query("SELECT ".$var." FROM users WHERE username = '".$username."' LIMIT 1");
$result = $query->fetch_assoc(); // fetch it first
return $result[$var];
}
Then use your function:
echo $user->GetVar('rank', 'Liam', $mysqli);
Important Note: Since you're starting out, kindly check about prepared statements. Don't directly append user input on your query.