where to put php.ini exactly ?

Little precision, when I switch to devellopment environment, I get this :

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


Should I search rather in CI or rather to my hosting company ?

Thx !
PS : when I remove the php.ini, I have no error in dev env.