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This is a note of what to do when you got an error "Warning: session_start() " by installing CakePHP.
It is an error that there is no permission to the file in which to save the session.
If you can use the [session.save_path] settings in php.ini, you can be resolved by allowing the writing [/tmp] directory.
You should be careful in the rental server, so there is a case where php.ini is not available.
It is also possible to set the [session.save_path] in .htaccess.
I modified the file of [app / Config / core.php].
Part of the 219 line of the link(GitHub).
It is described in part commented out of line 181-217 for details.
//default Configure::write('Session', array( 'defaults' => 'php' ));
//After Modify Configure::write('Session', array( 'defaults' => 'cake' ));
Change permissions of folder
After, I was changed to  from  the permission of [/app/tmp/sessions] folder.
php - CakePHP: Permission denied (13) - Stack Overflow