how to use mysqli_query() Function ?

mysqli_query($dbcon,"SELECT * FROM tablename ");

$dbcon=mysqli_connect("server name is here","database username is here ","database password is here ","database name is here");

// establish connection with Databse 
$dbcon=mysqli_connect("server name is here","database username is here ","database password is here ","database name is here"); 

//check if any problem in connection or not 
if (mysqli_connect_errno())
  {
  echo " MySQL Connection Error :  " . mysqli_connect_error();
  }

// run query it return instance which required perform fetch data operation
mysqli_query($dbcon,"SELECT * FROM tablename ");