function selectQuery($query,$user_input=array()){

	$s='';
	if(count($user_input)>0){
		foreach( $user_input as $input ){
			$s.='s';
		}
	}
	$select=$this->mysqli->prepare($query);
	$select->bind_param($s,$user_input); //this is the line where the array is going in and apparently is not possible.
	$select->execute();
}
$query='SELECT * FROM users where username=?';
$user_input=array('test');
$test->selectQuery($query,$user_input);