在第9行的/home1/ksuexpre/public_html/conn.php中调用boolean上的成员函数set_charset（）

my project was working well, but suddenly there is an error shown says:

"Call to a member function set_charset() on boolean in /home1/express/public_html/conn.php on line 9"

this is my conn.php

<?php
$db_name = "";
$server_name = "";
$username = "";
$password = "";

// Create connection
$conn = mysqli_connect($server_name, $username, $password, $db_name);
$conn->set_charset("utf8"); `


if (mysqli_connect_errno()) {
    printf("Connect failed: %s
", mysqli_connect_error());
    exit();
}

?>

you need to check if you have mysqli object before setting the charset

change this line

$conn = mysqli_connect($server_name, $username, $password, $db_name);
$conn->set_charset("utf8"); `

 if (mysqli_connect_errno()) {  
     printf("Connect failed: %s
", mysqli_connect_error()); 
     exit();
 }

to

$conn = mysqli_connect($server_name, $username, $password, $db_name);


 if (mysqli_connect_errno()) {  
     printf("Connect failed: %s
", mysqli_connect_error()); 
     exit();
 }
$conn->set_charset("utf8"); `