php - Fatal error: Call to a member function connect() on null -

trying include function "overlay" config.php other php pages.

index.php:

<?php include("./config/config.php"); over_lay(); ?> //// html code //// 

the error is:

fatal error: call member function connect() on null in ...\config\config.php on line 182 

part of config.php:

include("./config/mysql.php"); function over_lay() { $mysqli->connect(); ?> //// html code partly mysqli requests //// <?php   $mysqli->close(); } ?> 

and mysql.php:

<?php function connect()  { $mysqli = new mysqli("localhost","root","","database_name"); $mysqli->query("set names utf8"); return $mysqli; } ?> 

i'm testing on openserver , of course have database.

call function inside over_lay

include("./config/mysql.php"); function over_lay() {     $mysqli = connect();      ?>      //// html code partly mysqli requests ////      <?php      $mysqli->close();  } 

update changed

$mysqli->connect();

to

$mysqli = connect();