Add/expand comments for PDO handlers

Author: Girgias

ext/pdo/pdo_dbh.c

@@ -600,8 +600,7 @@ PHP_METHOD(PDO, beginTransaction)
 	}
 
 	if (!dbh->methods->begin) {
-		/* TODO: this should be an exception; see the auto-commit mode
-		 * comments below */
+		/* Throw an exception when the driver does not support transactions */
 		zend_throw_exception_ex(php_pdo_get_exception(), 0, "This driver doesn't support transactions");
 		RETURN_THROWS();
 	}
@@ -904,6 +903,8 @@ PHP_METHOD(PDO, getAttribute)
 			RETURN_FALSE;
 
 		default:
+			/* No error state, just return as the return_value has been assigned
+			 * by the get_attribute handler */
 			return;
 	}
 }

ext/pdo/php_pdo_driver.h

@@ -255,7 +255,11 @@ typedef char *(*pdo_dbh_last_id_func)(pdo_dbh_t *dbh, const char *name, size_t *
  *   specific data ...  */
 typedef	int (*pdo_dbh_fetch_error_func)(pdo_dbh_t *dbh, pdo_stmt_t *stmt, zval *info);
 
-/* fetching of attributes */
+/* fetching of attributes
+ * There are 3 return states:
+ * * -1 for errors while retrieving a valid attribute
+ * * 0 for attempting to retrieve an attribute which is not supported by the driver
+ * * any other value for success, *val must be set to the attribute value */
 typedef int (*pdo_dbh_get_attr_func)(pdo_dbh_t *dbh, zend_long attr, zval *val);
 
 /* checking/pinging persistent connections; return SUCCESS if the connection