## How it works

This describes the background and inner workings of the mysqlnd_uh extension.

Two classes are provided by the extension: MysqlndUhConnection and MysqlndUhPreparedStatement. MysqlndUhConnection lets you access almost all methods of the `mysqlnd` internal `connection` class. The latter exposes some selected methods of the `mysqlnd` internal `statement` class. For example, MysqlndUhConnection::connect() maps to the `mysqlnd` library C function `mysqlnd_conn__connect`.

As a mysqlnd plugin, the PECL/mysqlnd_uh extension replaces `mysqlnd` library C functions with its own functions. Whenever a PHP MySQL extension compiled to use `mysqlnd` calls a mysqlnd function, the functions installed by the plugin are executed instead of the original `mysqlnd` ones. For example, mysqli_connect() invokes `mysqlnd_conn__connect`, so the connect function installed by PECL/mysqlnd_uh will be called. The functions installed by PECL/mysqlnd_uh are the methods of the built-in classes.

The built-in PHP classes and their methods do nothing but call their `mysqlnd` C library counterparts, to behave exactly like the original `mysqlnd` function they replace. The code below illustrates in pseudo-code what the extension does.

Example #1 Pseudo-code: what a built-in class does

    class MysqlndUhConnection {
      public function connect(($conn, $host, $user, $passwd, $db, $port, $socket, $mysql_flags) {
        MYSQLND* c_mysqlnd_connection = convert_from_php_to_c($conn);
        ...
        return call_c_function(mysqlnd_conn__connect(c_mysqlnd_connection, ...));
      }
    }

The build-in classes behave like a transparent proxy. It is possible for you to replace the proxy with your own. This is done by subclassing MysqlndUhConnection or MysqlndUhPreparedStatement to extend the functionality of the proxy, followed by registering a new proxy object. Proxy objects are installed by mysqlnd_uh_set_connection_proxy() and mysqlnd_uh_set_statement_proxy().

Example #2 Installing a proxy

`<?php
class proxy extends MysqlndUhConnection {
public function connect($res, $host, $user, $passwd, $db, $port, $socket, $mysql_flags) {
printf("%s(%s)\n", __METHOD__, var_export(func_get_args(), true));
$ret = parent::connect($res, $host, $user, $passwd, $db, $port, $socket, $mysql_flags);
printf("%s returns %s\n", __METHOD__, var_export($ret, true));
return $ret;
}
}
mysqlnd_uh_set_connection_proxy(new proxy());
$mysqli = new mysqli("localhost", "root", "", "test");
?>`

The above example will output:

    proxy::connect(array (
      0 => NULL,
      1 => 'localhost',
      2 => 'root',
      3 => '',
      4 => 'test',
      5 => 3306,
      6 => NULL,
      7 => 131072,
    ))
    proxy::connect returns true