Using singleton pattern for a MySQL connection
First, on your premises.


First of all, no pattern would prevent PHP from making a connection to MySQL database on every web api call. Because PHP will die between different API calls, along with all its singletons, connections and any other stuff. So all you can prevent is reconnecting the MySQL database on every call to a database connection class.
No, it is frowned upon using singleton pattern to prevent reconnecting the MySQL database on every call to a database connection class. A more accepted approach is Dependency Injection. 
Funny enough, due to a typo, this class would not prevent reconnecting the mysql. Had you error_reporting set to E_ALL, PHP would have signaled that you are trying to use a non-existent variable $connection every time getInstance() is called. Surely you wanted to call it as $this-&gt;connection instead.


Now to the code.   

The intention is very good, especially I like getResult() and execute() methods that allow you to avoid that bind param hassle. However, there is evidently a duplicated code. Why not to make execute to return $statement? It will let you make getResult() as simple as

public function getResult(string $sql, iterable $params) : mysqli_result
{
    return $this-&gt;execute($sql, $params)-&gt;get_result();
}


And finally. I just noticed, halfway the review process, that your code is off topic, as it simply doesn't work. Mysqli is not PDO, you cannot bind your parameters in a loop. So you need to rework it. In order to help, here are two my articles:


How to properly connect to Mysql database using mysqli that will show you important options missed in your connection code
Mysqli made simple to show you how to bind parameters for mysqli dynamically