大神指导Fatal error: Class 'mysqli' not found in D:\PHP\Pack

Fatal error: Class 'mysqli' not found in D:\PHP\Package\htdocs\shujuku.php on line 8
这是怎么回事啊？
源码
<?php
$servername = "localhost";
$username = "root";
$password = "1a2a3a4a5a";
$dbname = "test";
// 创建连接
$conn = new mysqli($servername, $username, $password, $dbname);
// 检测连接
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}
$sql = "SELECT id, firstname, lastname FROM MyGuests";
$result = $conn->query($sql);
if ($result->num_rows > 0) {
// 输出每行数据
while($row = $result->fetch_assoc()) {
echo "<br> id: ". $row["id"]. " - Name: ". $row["firstname"]. " " . $row["lastname"];
}
} else {
echo "0 results";
}
$conn->close();
?>