$string="1,2,3,4,5";
$array=array_map('intval', explode(',', $string));
$array = implode("','",$array);
$query=mysqli_query($conn, "SELECT name FROM users WHERE id IN ('".$array."')");
注意:構文は次のとおりです:
SELECT * FROM table WHERE column IN('value1','value2','value3')