where to put php.ini exactly ?

A PHP Error was encountered

Severity: Warning

Message: mkdir(): Invalid path

Filename: drivers/Session_files_driver.php

Line Number: 118

Backtrace:

File: /homepages/23/d572140987/htdocs/myxxx/application/core/MY_Controller.php
Line: 173
Function: __construct

File: /homepages/23/d572140987/htdocs/myxxx/application/controllers/Index.php
Line: 8
Function: __construct

File: /homepages/23/d572140987/htdocs/myxxx/index.php
Line: 316
Function: require_once

A PHP Error was encountered

Severity: Warning

Message: Cannot modify header information - headers already sent by (output started at /homepages/23/d572140987/htdocs/myxxx/system/core/Exceptions.php:272)

Filename: core/Common.php

Line Number: 568

Backtrace:

An uncaught Exception was encountered

Type: Exception

Message: Session: Configured save path '' is not a directory, doesn't exist or cannot be created.

Filename: /homepages/23/d572140987/htdocs/myxxx/system/libraries/Session/drivers/Session_files_driver.php

Line Number: 120

Backtrace:

File: /homepages/23/d572140987/htdocs/myxxx/application/core/MY_Controller.php
Line: 173
Function: __construct

File: /homepages/23/d572140987/htdocs/myxxx/application/controllers/Index.php
Line: 8
Function: __construct

File: /homepages/23/d572140987/htdocs/myxxx/index.php
Line: 316
Function: require_once

Provide an absolute path in application/config/config.php file

$config['sess_save_path'] = NULL; e.g. replace NULL with APPPATH.'sess/';//sess folder must be present