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Showing content from https://www.yiiframework.com/doc/api/2.0/yii-db-connection below:

Connection, yii\db\Connection | API Documentation for Yii 2.0

Connection represents a connection to a database via PDO.

Connection works together with yii\db\Command, yii\db\DataReader and yii\db\Transaction to provide data access to various DBMS in a common set of APIs. They are a thin wrapper of the PDO PHP extension.

Connection supports database replication and read-write splitting. In particular, a Connection component can be configured with multiple $masters and $slaves. It will do load balancing and failover by choosing appropriate servers. It will also automatically direct read operations to the slaves and write operations to the masters.

To establish a DB connection, set $dsn, $username and $password, and then call open() to connect to the database server. The current state of the connection can be checked using $isActive.

The following example shows how to create a Connection instance and establish the DB connection:

$connection = new \yii\db\Connection([
    'dsn' => $dsn,
    'username' => $username,
    'password' => $password,
]);
$connection->open();

After the DB connection is established, one can execute SQL statements like the following:

$command = $connection->createCommand('SELECT * FROM post');
$posts = $command->queryAll();
$command = $connection->createCommand('UPDATE post SET status=1');
$command->execute();

One can also do prepared SQL execution and bind parameters to the prepared SQL. When the parameters are coming from user input, you should use this approach to prevent SQL injection attacks. The following is an example:

$command = $connection->createCommand('SELECT * FROM post WHERE id=:id');
$command->bindValue(':id', $_GET['id']);
$post = $command->query();

For more information about how to perform various DB queries, please refer to yii\db\Command.

If the underlying DBMS supports transactions, you can perform transactional SQL queries like the following:

$transaction = $connection->beginTransaction();
try {
    $connection->createCommand($sql1)->execute();
    $connection->createCommand($sql2)->execute();
    
    $transaction->commit();
} catch (Exception $e) {
    $transaction->rollBack();
}

You also can use shortcut for the above like the following:

$connection->transaction(function () {
    $order = new Order($customer);
    $order->save();
    $order->addItems($items);
});

If needed you can pass transaction isolation level as a second parameter:

$connection->transaction(function (Connection $db) {
    
}, Transaction::READ_UNCOMMITTED);

Connection is often used as an application component and configured in the application configuration like the following:

'components' => [
    'db' => [
        'class' => '\yii\db\Connection',
        'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
        'username' => 'root',
        'password' => '',
        'charset' => 'utf8',
    ],
],

Hide inherited properties

PDO attributes (name => value) that should be set when calling open() to establish a DB connection. Please refer to the PHP manual for details about available attributes.

The charset used for database connection. The property is only used for MySQL, PostgreSQL and CUBRID databases. Defaults to null, meaning using default charset as configured by the database.

For Oracle Database, the charset must be specified in the $dsn, for example for UTF-8 by appending ;charset=UTF-8 to the DSN string.

The same applies for if you're using GBK or BIG5 charset with MySQL, then it's highly recommended to specify charset via $dsn like 'mysql:dbname=mydatabase;host=127.0.0.1;charset=GBK;'.

Deprecated since 2.0.14. Use $commandMap for precise configuration.

The class used to create new database yii\db\Command objects. If you want to extend the yii\db\Command class, you may configure this property to use your extended version of the class. Since version 2.0.14 $commandMap is used if this property is set to its default value.

See also:

The password for establishing DB connection. Defaults to null meaning no password to use.

The PHP PDO instance associated with this DB connection. This property is mainly managed by open() and close() methods. When a DB connection is active, this property will represent a PDO instance; otherwise, it will be null.

See also:

The schema information for the database opened by this connection.

The cache object or the ID of the cache application component that is used to cache the table metadata.

See also:

openFromPoolSequentially() for details about the failover behavior.
$serverRetryInterval

Server version as a string.

The currently active slave connection. null is returned if there is no slave available and $fallbackToMaster is false.

The configuration that should be merged with every slave configuration listed in $slaves. For example,

[
    'username' => 'slave',
    'password' => 'slave',
    'attributes' => [
        
        PDO::ATTR_TIMEOUT => 10,
    ],
]

The PDO instance for the currently active slave connection. null is returned if no slave connection is available and $fallbackToMaster is false.

List of slave connection configurations. Each configuration is used to create a slave DB connection. When $enableSlaves is true, one of these configurations will be chosen and used to create a DB connection for performing read queries only.

See also:

The common prefix or suffix for table names. If a table name is given as {{%TableName}}, then the percentage character % will be replaced with this property value. For example, {{%post}} becomes {{tbl_post}}.

The currently active transaction. Null if no active transaction.

The username for establishing DB connection. Defaults to null meaning no username to use.

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Defined in: yii\base\Component::__call()

Calls the named method which is not a class method.

This method will check if any attached behavior has the named method and will execute it if available.

Do not call this method directly as it is a PHP magic method that will be implicitly called when an unknown method is being invoked.