`<?php
$servername = "localhost";
$usrname   = "root";
$pwd = "";
//$db = "test_db";
// connect to the database
$conn= mysqli_connect($servername, $usrname, $pwd);
if (!$conn){
    die('connection failed' . mysqli_connect_error());
}
// Create database
$sql = "CREATE DATABASE IF NOT EXISTS test_db";
    if (mysqli_query($conn, $sql)) {
    echo "Database created successfully<br>";
} else {
  echo "Error creating database: " . mysqli_error($conn);
}
?>`

你的代码很好，你只需要从mysqli_connect（）中删除$db。因为你请求连接“test_db”到这个已经不存在的数据库。

<?php
// Connect to MySQL
$link = mysql_connect('localhost', 'mysql_user', 'mysql_password');
if (!$link) {
    die('Could not connect: ' . mysql_error());
}

// Make my_db the current database
$db_selected = mysql_select_db('my_db', $link);

if (!$db_selected) {
  // If we couldn't, then it either doesn't exist, or we can't see it.
  $sql = 'CREATE DATABASE my_db';

  if (mysql_query($sql, $link)) {
      echo "Database my_db created successfully\n";
  } else {
      echo 'Error creating database: ' . mysql_error() . "\n";
  }
}

mysql_close($link);
?>

参考How do I create a database if it doesn't exist, using PHP?

如果您有任何问题，请与我们联系。
您正尝试连接到最初不存在的数据库，因此收到致命错误。
尝试使用try catch块创建新数据库

try{
$conn= mysqli_connect($servername, $usrname, $pwd,$db);
}cath(Exception $e){
//your code
}