Drupal MySQL character set converter to utf8mb4

charset-converter.php

<?php

// Helper for https://www.drupal.org/node/2488180
// Run: drush src charset-converter.php

class CharsetConverter {

  /**
   * Character set.
   * @var string
   */
  private $charset = 'utf8';

  /**
   * Collation.
   * @var string
   */
  private $collation = 'utf8_unicode_ci';

  public function __construct($charset = NULL, $collation = NULL) {
    $this->charset = !$charset ?: 'utf8';
    $this->collation = !$collation ?: 'utf8_unicode_ci';
  }

  /**
   * Convert the MySQL drupal databases character set and collation.
   *
   * @param array $databases
   *   The Drupal 7 database array.
   */
  public function convert(array $databases) {
    foreach ($databases as $database_key => $database_values) {
      foreach ($database_values as $target => $database) {
        // MySQL, mysqli, mysqlnd, etc.
        if (strpos($database['driver'], 'mysql') === 0) {
          drush_print('Target MySQL database: ' . $database_key . ':' . $target);
          // Connect to next database.
          db_set_active($database_key);

          // Check the database type is mysql.
          $db_type = Database::getConnection()->databaseType();
          // Skip if not MySQL.
          if ($db_type !== 'mysql') {
            continue;
          }

          // @todo check if large prefixes is enabled.

          // Get the charset and collation we are going to change to from the config.
          $this->charset = (!empty($database['charset'])) ? $database['charset'] : 'utf8';
          $this->collation = (!empty($database['collation'])) ? $database['collation'] : 'utf8_unicode_ci';

          // For each database:
          $this->convertDatabase($database['database']);

          // For each table in the database.
          $this->convertTables();

          // // For each column: (Doesn't seem to be needed)
          // ALTER TABLE table_name CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
        }
      }
    }
    // Reset connection to default.
    db_set_active();
  }

  /**
   * @param string
   *   Database name.
   *
   * @param string $charset
   *   (Optional) The character set.
   * @param string $collation
   *   (Optional) The collation.
   *
   * @return bool
   *   success|failure.
   */
  public function convertDatabase($database_name, $charset = NULL, $collation = NULL) {
    drush_print('Converting database: ' . $database_name);
    $sql = "ALTER DATABASE " . $database_name . " CHARACTER SET = :charset COLLATE = :collation;";
    return db_query($sql, array(
      ':charset' => $charset ?: $this->charset,
      ':collation' => $collation ?: $this->collation,
    ));
  }

  /**
   * Converts all the tables defined by drupal_get_schema().
   *
   * @param string $charset
   *   (Optional) The character set.
   * @param string $collation
   *   (Optional) The collation.
   *
   * @return bool
   *   success|failure.
   */
  public function convertTables($charset = NULL, $collation = NULL) {
    // For each table:
    // Deal only with Drupal managed tables.
    $schema = drupal_get_schema();
    $table_names = array_keys($schema);
    sort($table_names);
    foreach (array_keys($schema) as $table_name) {
      if (!db_table_exists($table_name)) {
        continue;
      }
      $this->convertTable($table_name, $charset, $collation);
    }
  }

  /**
   * Converts a table to a desired character set and collation.
   *
   * @param string $table_name
   *  The database table name.
   * @param string $charset
   *   (Optional) The character set.
   * @param string $collation
   *   (Optional) The collation.
   *
   * @return bool
   *   success|failure.
   */
  public function convertTable($table_name, $charset = NULL, $collation = NULL) {
    $sql = "ALTER TABLE {" . $table_name . "} CHARACTER SET = :charset COLLATE = :collation";
    drush_print('Converting table: ' . $table_name);
    $result = db_query($sql, array(
      ':charset' => $charset ?: $this->charset,
      ':collation' => $collation ?: $this->collation,
    ));
    db_query("ALTER TABLE {" . $table_name . "} ROW_FORMAT=DYNAMIC");
    db_query("REPAIR TABLE {" . $table_name . "}");
    db_query("OPTIMIZE TABLE {" . $table_name . "}");

    $this->convertTableFields($table_name, $charset, $collation);

    return $result;
  }

  /**
   * Converts a table's field to a desired character set and collation.
   *
   * @param string $table_name
   *  The database table name.
   * @param string $charset
   *   (Optional) The character set.
   * @param string $collation
   *   (Optional) The collation.
   *
   * @return bool
   *   success|failure.
   */
  public function convertTableFields($table_name, $charset = NULL, $collation = NULL) {
    $table_name = Database::getConnection()->prefixTables('{' . db_escape_table($table_name) . '}');
    $results = db_query("SHOW FULL FIELDS FROM {" . $table_name . "}")->fetchAllAssoc('Field');
    $charset = $charset ?: $this->charset;
    $collation = $collation ?: $this->collation;
    foreach ($results as $row) {
      // Skip fields that don't have collation, their probably int or something.
      // or if we are using that collation for this field already save a query
      // or is not binary.
      if (!$row->Collation || $row->Collation === $collation || strpos($row->Collation, '_bin') !== FALSE) {
        continue;
      }
      $sql = "ALTER TABLE {" . $table_name . "} MODIFY " . $row->Field . " " . $row->Type . " CHARACTER SET :charset COLLATE :collation";
      db_query($sql, array(
        ':charset' => $charset,
        ':collation' => $collation,
      ));
    }
  }

}

$converter = new CharsetConverter();
global $databases;
$converter->convert($databases);

This loses default value, not null and comment definitions because of the MODIFY column statement :(

See https://www.drupal.org/node/2781545

Anyone wanting this functionality, consider using beta2 or later of https://www.drupal.org/project/utf8mb4_convert instead