class Connection

1. 8.x drupal/core/lib/Drupal/Core/Database/Connection.php \Drupal\Core\Database\Connection
2. 8.x drupal/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php \Drupal\Core\Database\Driver\mysql\Connection
3. 8.x drupal/core/lib/Drupal/Core/Database/Driver/sqlite/Connection.php \Drupal\Core\Database\Driver\sqlite\Connection
4. 8.x drupal/core/lib/Drupal/Core/Database/Driver/pgsql/Connection.php \Drupal\Core\Database\Driver\pgsql\Connection

Tasks.php in drupal/core/lib/Drupal/Core/Database/Driver/mysql/Install/Tasks.php

Definition of Drupal\Core\Database\Driver\mysql\Install\Tasks

Response::getConnection in drupal/core/vendor/guzzle/http/Guzzle/Http/Message/Response.php

Get the Connection HTTP header

class Connection extends DatabaseConnection {

  /**
   * Error code for "Unknown database" error.
   */
  const DATABASE_NOT_FOUND = 1049;

  /**
   * Flag to indicate if the cleanup function in __destruct() should run.
   *
   * @var boolean
   */
  protected $needsCleanup = FALSE;

  /**
   * Constructs a Connection object.
   */
  public function __construct(PDO $connection, array $connection_options = array()) {
    parent::__construct($connection, $connection_options);

    // This driver defaults to transaction support, except if explicitly passed FALSE.
    $this->transactionSupport = !isset($connection_options['transactions']) || $connection_options['transactions'] !== FALSE;

    // MySQL never supports transactional DDL.
    $this->transactionalDDLSupport = FALSE;
    $this->connectionOptions = $connection_options;
  }

  /**
   * {@inheritdoc}
   */
  public static function open(array &$connection_options = array()) {

    // The DSN should use either a socket or a host/port.
    if (isset($connection_options['unix_socket'])) {
      $dsn = 'mysql:unix_socket=' . $connection_options['unix_socket'];
    }
    else {

      // Default to TCP connection on port 3306.
      $dsn = 'mysql:host=' . $connection_options['host'] . ';port=' . (empty($connection_options['port']) ? 3306 : $connection_options['port']);
    }
    if (!empty($connection_options['database'])) {
      $dsn .= ';dbname=' . $connection_options['database'];
    }

    // Allow PDO options to be overridden.
    $connection_options += array(
      'pdo' => array(),
    );
    $connection_options['pdo'] += array(
      PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
      // So we don't have to mess around with cursors and unbuffered queries by default.
      PDO::MYSQL_ATTR_USE_BUFFERED_QUERY => TRUE,
      // Make sure MySQL returns all matched rows on update queries including
      // rows that actually didn't have to be updated because the values didn't
      // change. This matches common behaviour among other database systems.
      PDO::MYSQL_ATTR_FOUND_ROWS => TRUE,
      // Because MySQL's prepared statements skip the query cache, because it's dumb.
      PDO::ATTR_EMULATE_PREPARES => TRUE,
    );
    $pdo = new PDO($dsn, $connection_options['username'], $connection_options['password'], $connection_options['pdo']);

    // Force MySQL to use the UTF-8 character set. Also set the collation, if a
    // certain one has been set; otherwise, MySQL defaults to 'utf8_general_ci'
    // for UTF-8.
    if (!empty($connection_options['collation'])) {
      $pdo
        ->exec('SET NAMES utf8 COLLATE ' . $connection_options['collation']);
    }
    else {
      $pdo
        ->exec('SET NAMES utf8');
    }

    // Set MySQL init_commands if not already defined.  Default Drupal's MySQL
    // behavior to conform more closely to SQL standards.  This allows Drupal
    // to run almost seamlessly on many different kinds of database systems.
    // These settings force MySQL to behave the same as postgresql, or sqlite
    // in regards to syntax interpretation and invalid data handling.  See
    // http://drupal.org/node/344575 for further discussion. Also, as MySQL 5.5
    // changed the meaning of TRADITIONAL we need to spell out the modes one by
    // one.
    $connection_options += array(
      'init_commands' => array(),
    );
    $connection_options['init_commands'] += array(
      'sql_mode' => "SET sql_mode = 'ANSI,STRICT_TRANS_TABLES,STRICT_ALL_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER'",
    );

    // Set connection options.
    $pdo
      ->exec(implode('; ', $connection_options['init_commands']));
    return $pdo;
  }
  public function __destruct() {
    if ($this->needsCleanup) {
      $this
        ->nextIdDelete();
    }
  }
  public function queryRange($query, $from, $count, array $args = array(), array $options = array()) {
    return $this
      ->query($query . ' LIMIT ' . (int) $from . ', ' . (int) $count, $args, $options);
  }
  public function queryTemporary($query, array $args = array(), array $options = array()) {
    $tablename = $this
      ->generateTemporaryTableName();
    $this
      ->query(preg_replace('/^SELECT/i', 'CREATE TEMPORARY TABLE {' . $tablename . '} Engine=MEMORY SELECT', $query), $args, $options);
    return $tablename;
  }
  public function driver() {
    return 'mysql';
  }
  public function databaseType() {
    return 'mysql';
  }

  /**
   * Overrides \Drupal\Core\Database\Connection::createDatabase().
   *
   * @param string $database
   *   The name of the database to create.
   *
   * @throws DatabaseNotFoundException
   */
  public function createDatabase($database) {

    // Escape the database name.
    $database = Database::getConnection()
      ->escapeDatabase($database);
    try {

      // Create the database and set it as active.
      $this->connection
        ->exec("CREATE DATABASE {$database}");
      $this->connection
        ->exec("USE {$database}");
    } catch (\Exception $e) {
      throw new DatabaseNotFoundException($e
        ->getMessage());
    }
  }
  public function mapConditionOperator($operator) {

    // We don't want to override any of the defaults.
    return NULL;
  }
  public function nextId($existing_id = 0) {
    $new_id = $this
      ->query('INSERT INTO {sequences} () VALUES ()', array(), array(
      'return' => Database::RETURN_INSERT_ID,
    ));

    // This should only happen after an import or similar event.
    if ($existing_id >= $new_id) {

      // If we INSERT a value manually into the sequences table, on the next
      // INSERT, MySQL will generate a larger value. However, there is no way
      // of knowing whether this value already exists in the table. MySQL
      // provides an INSERT IGNORE which would work, but that can mask problems
      // other than duplicate keys. Instead, we use INSERT ... ON DUPLICATE KEY
      // UPDATE in such a way that the UPDATE does not do anything. This way,
      // duplicate keys do not generate errors but everything else does.
      $this
        ->query('INSERT INTO {sequences} (value) VALUES (:value) ON DUPLICATE KEY UPDATE value = value', array(
        ':value' => $existing_id,
      ));
      $new_id = $this
        ->query('INSERT INTO {sequences} () VALUES ()', array(), array(
        'return' => Database::RETURN_INSERT_ID,
      ));
    }
    $this->needsCleanup = TRUE;
    return $new_id;
  }
  public function nextIdDelete() {

    // While we want to clean up the table to keep it up from occupying too
    // much storage and memory, we must keep the highest value in the table
    // because InnoDB  uses an in-memory auto-increment counter as long as the
    // server runs. When the server is stopped and restarted, InnoDB
    // reinitializes the counter for each table for the first INSERT to the
    // table based solely on values from the table so deleting all values would
    // be a problem in this case. Also, TRUNCATE resets the auto increment
    // counter.
    try {
      $max_id = $this
        ->query('SELECT MAX(value) FROM {sequences}')
        ->fetchField();

      // We know we are using MySQL here, no need for the slower db_delete().
      $this
        ->query('DELETE FROM {sequences} WHERE value < :value', array(
        ':value' => $max_id,
      ));
    } catch (DatabaseException $e) {
    }
  }

  /**
   * Overridden to work around issues to MySQL not supporting transactional DDL.
   */
  protected function popCommittableTransactions() {

    // Commit all the committable layers.
    foreach (array_reverse($this->transactionLayers) as $name => $active) {

      // Stop once we found an active transaction.
      if ($active) {
        break;
      }

      // If there are no more layers left then we should commit.
      unset($this->transactionLayers[$name]);
      if (empty($this->transactionLayers)) {
        if (!$this->connection
          ->commit()) {
          throw new TransactionCommitFailedException();
        }
      }
      else {

        // Attempt to release this savepoint in the standard way.
        try {
          $this
            ->query('RELEASE SAVEPOINT ' . $name);
        } catch (DatabaseExceptionWrapper $e) {

          // However, in MySQL (InnoDB), savepoints are automatically committed
          // when tables are altered or created (DDL transactions are not
          // supported). This can cause exceptions due to trying to release
          // savepoints which no longer exist.
          //
          // To avoid exceptions when no actual error has occurred, we silently
          // succeed for MySQL error code 1305 ("SAVEPOINT does not exist").
          if ($e
            ->getPrevious()->errorInfo[1] == '1305') {

            // If one SAVEPOINT was released automatically, then all were.
            // Therefore, clean the transaction stack.
            $this->transactionLayers = array();

            // We also have to explain to PDO that the transaction stack has
            // been cleaned-up.
            $this->connection
              ->commit();
          }
          else {
            throw $e;
          }
        }
      }
    }
  }

}