It should be noted that PDO::FETCH_CLASS will call the constructor *after* setting the values (or calling __set).The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.
   See also PDO::ATTR_CURSOR_NAME.
  
PDO::FETCH_ORI_NEXT
     (int)
    PDO::FETCH_ORI_PRIOR
     (int)
    PDO::FETCH_ORI_FIRST
     (int)
    PDO::FETCH_ORI_LAST
     (int)
    PDO::FETCH_ORI_ABS
     (int)
    PDO::FETCH_ORI_REL
     (int)
    PDO::CURSOR_FWDONLY
     (int)
    PDO::CURSOR_SCROLL
     (int)
    PDO::FETCH_ORI_* constants to control the rows fetched from the result set.
     
    PDO::PARAM_BOOL
     (int)
    PDO::PARAM_NULL
     (int)
    PDO::PARAM_INT
     (int)
    PDO::PARAM_STR
     (int)
    PDO::PARAM_STR_NATL
     (int)
    PDO::PARAM_STR_CHAR
     (int)
    PDO::PARAM_LOB
     (int)
    PDO::PARAM_STMT
     (int)
    PDO::PARAM_INPUT_OUTPUT
     (int)
    PDO::ATTR_AUTOCOMMIT
     (int)
    false, PDO attempts to disable autocommit so that the
      connection begins a transaction.
     
    PDO::ATTR_PREFETCH
     (int)
    PDO::ATTR_TIMEOUT
     (int)
    PDO::ATTR_ERRMODE
     (int)
    PDO::ATTR_SERVER_VERSION
     (int)
    PDO::ATTR_CLIENT_VERSION
     (int)
    PDO::ATTR_SERVER_INFO
     (int)
    PDO::ATTR_CONNECTION_STATUS
     (int)
    PDO::ATTR_CASE
     (int)
    PDO::CASE_*
      constants.
     
    PDO::ATTR_CURSOR_NAME
     (int)
    PDO::ATTR_CURSOR
     (int)
    PDO::CURSOR_FWDONLY and
      PDO::CURSOR_SCROLL. Stick with
      PDO::CURSOR_FWDONLY unless you know that you need a
      scrollable cursor.
     
    PDO::ATTR_DRIVER_NAME
     (int)
    
Example #1 using PDO::ATTR_DRIVER_NAME
<?php
 if ($db->getAttribute(PDO::ATTR_DRIVER_NAME) == 'mysql') {
   echo "Running on mysql; doing something mysql specific here\n";
 }
 ?>PDO::ATTR_ORACLE_NULLS
     (int)
    PDO::ATTR_PERSISTENT
     (int)
    PDO::ATTR_STATEMENT_CLASS
     (int)
    PDO::ATTR_FETCH_CATALOG_NAMES
     (int)
    PDO::ATTR_FETCH_TABLE_NAMES
     (int)
    PDO::ATTR_STRINGIFY_FETCHES
     (int)
    null) to be treated as strings.
      null values remain unchanged unless PDO::ATTR_ORACLE_NULLS
      is set to PDO::NULL_TO_STRING.
     
    PDO::ATTR_MAX_COLUMN_LEN
     (int)
    PDO::ATTR_DEFAULT_FETCH_MODE
     (int)
    PDO::ATTR_EMULATE_PREPARES
     (int)
    PDO::ATTR_DEFAULT_STR_PARAM
     (int)
    PDO::PARAM_STR_NATL
      and PDO::PARAM_STR_CHAR.
     
     
      Available since PHP 7.2.0.
     
    PDO::ERRMODE_SILENT
     (int)
    PDO::ERRMODE_WARNING
     (int)
    E_WARNING message if an error occurs.
      See Errors and error handling
      for more information about this attribute.
     
    PDO::ERRMODE_EXCEPTION
     (int)
    PDO::CASE_NATURAL
     (int)
    PDO::CASE_LOWER
     (int)
    PDO::CASE_UPPER
     (int)
    PDO::NULL_NATURAL
     (int)
    PDO::NULL_EMPTY_STRING
     (int)
    PDO::NULL_TO_STRING
     (int)
    PDO::ERR_NONE
     (string)
    PDO::PARAM_EVT_ALLOC
     (int)
    PDO::PARAM_EVT_FREE
     (int)
    PDO::PARAM_EVT_EXEC_PRE
     (int)
    PDO::PARAM_EVT_EXEC_POST
     (int)
    PDO::PARAM_EVT_FETCH_PRE
     (int)
    PDO::PARAM_EVT_FETCH_POST
     (int)
    PDO::PARAM_EVT_NORMALIZE
     (int)
    PDO::SQLITE_DETERMINISTIC
     (int)
    It should be noted that PDO::FETCH_CLASS will call the constructor *after* setting the values (or calling __set).PDO::PARAM_STR_CHAR and PDO::PARAM_STR_NATL must be combined with PDO::PARAM_STR using bitwise-OR for parameter binding.
These flags control value quoting (e.g. PDO::quote) and, in some situations (see below), parameter binding  (e.g. PDO::bindParam, PDO::bindValue) to prefix string literals with N'' as defined in SQL-92.  As of PHP 7.3, only dblib and mysql support these flags.  For the mysql driver, the flags only affect parameter binding when PDO::ATTR_EMULATE_PREPARES is true (the default).
MySQL and MariaDB interpret string literals prefixed with N as being utf8 (not utf8mb4) regardless of `SET NAMES` or the charset parameter.  This can cause problems if the database/table/column charset is not utf8.  For example, in a database using utf8mb4, the query "SELECT * FROM table WHERE col = :param" and bindValue(":param", "\u{1F600}", PDO::PARAM_STR | PDO::PARAM_STR_NATL) will cause "PDOException: SQLSTATE[HY000]: General error: 1267 Illegal mix of collations (utf8mb4_general_ci,IMPLICIT) and (utf8_general_ci,COERCIBLE) for operation '='".  Using PDO::PARAM_STR without PDO::PARAM_STR_NATL and ensuring the charset DSN parameter is set correctly can avoid this issue.
See:
https://wiki.php.net/rfc/extended-string-types-for-pdo
https://mariadb.com/kb/en/library/string-literals/
https://dev.mysql.com/doc/refman/8.0/en/string-literals.htmlDefault value for \PDO::ATTR_TIMEOUT is 30 seconds.
Ref.: https://github.com/php/php-src/blob/PHP-7.1.0/ext/pdo_mysql/mysql_driver.c#L600To bind a float, use PDO::PARAM_STR with bindValue. You can skip the PDO::PARAM_STR because it's the default option. Binding with bindParam will change the type of the bound variable to a string, which can lead to type errors.PDO::FETCH_UNIQUE not only fetches the unique values, it also uses the first SQL column as array key result, what is very useful for create quickly an index, eg :
<?php
$sql = <<<SQL
    SELECT ALL
        c1, -- For result indexing
        c1, c2
    FROM (
        VALUES
            ROW('ID-1', 'Value 1'),
            ROW('ID-2', 'Value 2a'),
            ROW('ID-2', 'Value 2b'),
            ROW('ID-3', 'Value 3')
    ) AS t (c1, c2);
    SQL;
$result = $pdo->query($sql);
print_r($result->fetchAll(PDO::FETCH_UNIQUE));
/*
Gives :
ID-1 => [c1 => ID-1,  c2 => Value 1]
ID-2 => [c1 => ID-2b, c2 => Value 2b]
ID-3 => [c1 => ID-3,  c2 => Value 3]
*/
?>Reference to all the PDO::MYSQL_* constants is available in MySQL's documentation for the driver here: https://dev.mysql.com/doc/connectors/en/apis-php-pdo-mysql.html
Such as the ones I was looking for PDO::MYSQL_ATTR_SSL_CA and PDO::MYSQL_ATTR_SSL_CAPATH which are not listed on this page.