mysqli_num_rows函数用于获取结果集中的行数。使用方法如下：
1.创建数据库连接：
$conn = mysqli_connect("localhost", "username", "password", "database_name");
if (!$conn) {
die("连接失败: " . mysqli_connect_error());
}
2.执行SQL查询并获取结果集：
$sql = "SELECT * FROM table_name";
$result = mysqli_query($conn, $sql);
3.使用mysqli_num_rows函数获取结果集中的行数：
$row_count = mysqli_num_rows($result);
echo "行数：" . $row_count;
完整示例代码如下：
$conn = mysqli_connect("localhost", "username", "password", "database_name");
if (!$conn) {
die("连接失败: " . mysqli_connect_error());
}
$sql = "SELECT * FROM table_name";
$result = mysqli_query($conn, $sql);
$row_count = mysqli_num_rows($result);
echo "行数：" . $row_count;
mysqli_close($conn);
注意：在使用mysqli_num_rows函数之前，需要确保已经执行了SQL查询并获取了结果集。
