php mysqli query expects parameter 1 be to string

Question

I was coding a User database which calls an static method of find_all which calls an query method which in turn returns all the value of a database but i am finding trouble in d

Answer 1

Just read docs:
mixed

mysqli_query ( mysqli $link , string $query [, int $resultmode = MYSQLI_STORE_RESULT ] )    

You must change your arguments to

mysqli_query($this->connection, $sql)

Answer 2

When using mysqli in a procedural style, the first argument to mysqli_query must be a connection object

From the official documentation:

mixed mysqli_query ( mysqli $link , string $query [, int $resultmode = MYSQLI_STORE_RESULT ] )

So you need to change the order of the arguments, $this->connection goes first and $sql goes second:

$result = mysqli_query($this->connection, $sql);