mysqli扩展和持久化连接
mysqli扩展的持久化连接在PHP5.3中被引入。支持已经存在于PDO MYSQL 和ext/mysql中。持久化连接背后的思想是客户端进程和数据库之间的连接可以通过一个客户端进程来保持重用， 而不是多次的创建和销毁。这降低了每次需要创建一个新连接的开销，未使用的连接被缓存起来并且准备随时被重用。
不像mysql扩展，mysqli没有提供一个特殊的方法用于打开持久化连接。需要打开一个持久化连接时，你必须在 连接时在主机名前增加p:。
使用持久化连接的问题在于它们可能在客户端处于不可预知的状态。比如，一个表锁可能在客户端意外终止之前被激活。 一个新的客户端进程重用这个持久化连接就会"按照原样"得到这个连接。这样，一个新的客户端进程 为了更好的使用持久化连接，就需要做任何可能的清理工作，这样就增加了对程序员的负担。
mysqli扩展的持久化连接提供了内建的清理处理代码。mysqli 所做的清理工作包括：
回滚活动的事务
关闭并且删除临时表
对表解锁、
重置会话变量
关闭prepared语句（在PHP中经常发生）
关闭处理程序
释放通过 GET_LOCK()获得的锁
这确保了从连接池返回的持久化连接在客户端进程使用它之前处于干净的状态。
mysqli扩展通过自动的调用C-API函数mysql_change_user() 来完成这个清理工作。
自动清理的特性有优点也有缺点。优点是程序员不再需要担心附加的清理代码，因为它们会自动调用。然而缺点就是 代码可能会潜在的慢一点，因为每次从连接池返回一个连接都需要执行这些清理代码。
这个自动清理的代码可以通过在编译php时定义MYSQLI_NO_CHANGE_USER_ON_PCONNECT 来关闭。
mysqli扩展在使用Mysql Native Driver或Mysql Client Library（libmysql）时都支持持久化连接。
MySQLi类
代表PHP和Mysql数据库之间的一个连接。
mysqli::$affected_rows — Gets the number of affected rows in a previous MySQL operation
mysqli::autocommit — 打开或关闭本次数据库连接的自动命令提交事务模式
mysqli::begin_transaction — Starts a transaction
mysqli::change_user — Changes the user of the specified database connection
mysqli::character_set_name — 返回当前数据库连接的默认字符编码
mysqli::$client_info — Get MySQL client info
mysqli::$client_version — Returns the MySQL client version as a string
mysqli::close — 关闭先前打开的数据库连接
mysqli::commit — 提交一个事务
mysqli::$connect_errno — Returns the error code from last connect call
mysqli::$connect_error — Returns a string description of the last connect error
mysqli::__construct — Open a new connection to the MySQL server
mysqli::debug — Performs debugging operations
mysqli::dump_debug_info — 将调试信息输出到日志
mysqli::errno — 返回最近函数调用的错误代码
mysqli::$error_list — Returns a list of errors from the last command executed
mysqli::$error — Returns a string description of the last error
mysqli::$field_count — Returns the number of columns for the most recent query
mysqli::get_charset — Returns a character set object
mysqli::get_client_info — Get MySQL client info
mysqli_get_client_stats — Returns client per-process statistics
mysqli_get_client_version — Returns the MySQL client version as an integer
mysqli::get_connection_stats — Returns statistics about the client connection
mysqli::$host_info — Returns a string representing the type of connection used
mysqli::$protocol_version — Returns the version of the MySQL protocol used
mysqli::$server_info — Returns the version of the MySQL server
mysqli::$server_version — Returns the version of the MySQL server as an integer
mysqli::get_warnings — Get result of SHOW WARNINGS
mysqli::$info — Retrieves information about the most recently executed query
mysqli::init — Initializes MySQLi and returns a resource for use with mysqli_real_connect()
mysqli::$insert_id — Returns the auto generated id used in the last query
mysqli::kill — Asks the server to kill a MySQL thread
mysqli::more_results — Check if there are any more query results from a multi query
mysqli::multi_query — Performs a query on the database
mysqli::next_result — Prepare next result from multi_query
mysqli::options — Set options
mysqli::ping — Pings a server connection, or tries to reconnect if the connection has gone down
mysqli::poll — Poll connections
mysqli::prepare — Prepare an SQL statement for execution
mysqli::query — 对数据库执行一次查询
mysqli::real_connect — Opens a connection to a mysql server
mysqli::real_escape_string — Escapes special characters in a string for use in an SQL statement, taking into account the current charset of the connection
mysqli::real_query — 执行一个mysql查询
mysqli::reap_async_query — Get result from async query
mysqli::refresh — Refreshes
mysqli::release_savepoint — Rolls back a transaction to the named savepoint
mysqli::rollback — 回退当前事务
mysqli::rpl_query_type — Returns RPL query type
mysqli::savepoint — Set a named transaction savepoint
mysqli::select_db — 选择用于数据库查询的默认数据库
mysqli::send_query — 发送请求并返回结果
mysqli::set_charset — 设置默认字符编码
mysqli::set_local_infile_default — Unsets user defined handler for load local infile command
mysqli::set_local_infile_handler — Set callback function for LOAD DATA LOCAL INFILE command
mysqli::$sqlstate — Returns the SQLSTATE error from previous MySQL operation
mysqli::ssl_set — Used for establishing secure connections using SSL
mysqli::stat — Gets the current system status
mysqli::stmt_init — 初始化一条语句并返回一个用于mysqli_stmt_prepare(调用)的对象
mysqli::store_result — Transfers a result set from the last query
mysqli::$thread_id — Returns the thread ID for the current connection
mysqli::thread_safe — 返回是否是线程安全的
mysqli::use_result — Initiate a result set retrieval
mysqli::$warning_count — Returns the number of warnings from the last query for the given link
MySQLi_STMT类
代表一个prepared语句。
mysqli_stmt::$affected_rows — Returns the total number of rows changed, deleted, or inserted by the last executed statement
mysqli_stmt::attr_get — Used to get the current value of a statement attribute
mysqli_stmt::attr_set — Used to modify the behavior of a prepared statement
mysqli_stmt::bind_param — Binds variables to a prepared statement as parameters
mysqli_stmt::bind_result — Binds variables to a prepared statement for result storage
mysqli_stmt::close — Closes a prepared statement
mysqli_stmt::data_seek — Seeks to an arbitrary row in statement result set
mysqli_stmt::$errno — Returns the error code for the most recent statement call
mysqli_stmt::$error_list — Returns a list of errors from the last statement executed
mysqli_stmt::$error — Returns a string description for last statement error
mysqli_stmt::execute — Executes a prepared Query
mysqli_stmt::fetch — Fetch results from a prepared statement into the bound variables
mysqli_stmt::$field_count — Returns the number of field in the given statement
mysqli_stmt::free_result — Frees stored result memory for the given statement handle
mysqli_stmt::get_result — Gets a result set from a prepared statement
mysqli_stmt::get_warnings — Get result of SHOW WARNINGS
mysqli_stmt::$insert_id — Get the ID generated from the previous INSERT operation
mysqli_stmt::more_results — Check if there are more query results from a multiple query
mysqli_stmt::next_result — Reads the next result from a multiple query
mysqli_st