MySQL是一种开源的关系型数据库管理系统，广泛用于各种Web应用程序中。在论坛源码中，MySQL通常被用来存储用户、帖子、评论等信息。以下是一些常见的MySQL命令及其用法：

//连接MySQL数据库
$mysqli = new mysqli('localhost', 'username', 'password', 'database_name');
if ($mysqli->connect_errno) {
    die('Failed to connect to MySQL: (' . $mysqli->connect_errno . ') ' . $mysqli->connect_error);
}

//查询所有帖子
$sql = "SELECT * FROM posts";
$result = $mysqli->query($sql);
if ($result->num_rows > 0) {
    while ($row = $result->fetch_assoc()) {
        echo $row['post_title'];
    }
}

//插入新帖子
$sql = "INSERT INTO posts (post_title, post_content, user_id) VALUES ('New Post', 'This is a new post', 1)";
if ($mysqli->query($sql) === TRUE) {
    echo "New record created successfully";
} else {
    echo "Error: " . $sql . "
" . $mysqli->error;
}

//更新帖子内容
$sql = "UPDATE posts SET post_content='Updated post content' WHERE post_id=1";
if ($mysqli->query($sql) === TRUE) {
    echo "Record updated successfully";
} else {
    echo "Error updating record: " . $mysqli->error;
}

//删除帖子
$sql = "DELETE FROM posts WHERE post_id=1";
if ($mysqli->query($sql) === TRUE) {
    echo "Record deleted successfully";
} else {
    echo "Error deleting record: " . $mysqli->error;
}

//关闭数据库连接
$mysqli->close();

以上是一些最基础的MySQL命令和用法，论坛源码中还可能涉及到更高级的数据库操作。在编写论坛源码时，建议对MySQL有一定的了解和掌握，以确保程序的稳定性和安全性。