Did you change old urls with new one in database? You must have to change those.

Try 1. enable WP_DEBUG and check for errors. 2. rename plugins folder to plugins_backup and try again. And make sure theme's name are same. Do you have a URL where we can see the issue.

@Roberthue 1. for solution check my updated question. 2 already tried. 3. how to check theme's name are same or not. i have issue with dashboard login page. when i login page refresh only

The php session error you mentioned must also have a file path with it. Paste complete error message here.

Okay. First check there are no space or line break (new line) in wp-config.php & theme functions.php file. And the first thing is <?php. Second thing you need to do is, remove popup-with-fancybox plugin and try again.

Go into the sql database and find the wp_options table. Edit the column called active_plugins. Copy the entire value to a safe place. Delete it from the table and save it. This will deactive (not remove) all plugins that are installed. See if you are able to log in. You can copy the value back to the active_plugins column later or activate them via admin panel.

Is it still showing the same session_start error? If not, it must be unrelated to the login issue.

Add define('RELOCATE',true); to wp-config.php below your other define statements. It should update the new url in the database. See the Relocate Method in the codex.

Are you using the session_start() function anywhere in your functions.php file?

If you still have define('RELOCATE',true); in your wp-config.php, remove or comment it out. Save wp-config.php and go directly to your login url again.

The same error related to line 418 of wp-login.php is appearing?

The only things left to troubleshoot that I can think of: 1. Remove your custom functions.php file and see if this is the source of the conflict. 2. Compare your core files, particularly wp-login.php and pluggable.php to those found at WordPress Core for the version of WordPress you have installed.

You should only need to remove functions.php for as long as it takes to attempt to log in. Make sure you save a backup, of course.

@RyanLoremIpsum no use. Now installing new files from wp-include, wp-admin and wp-...files.

@toscho thanx

@toscho can u help me?

no time, sorry

then why u added me in chat box

because it is your question

Ok chat is finally working. For some reason I had to add cookie exceptions for stackexchange.com and stackauth.com because it considers them third-party cookies and site data

good

Anyway, he moved the chat here because it has been a long troubleshooting session and only the final answer needs to be posted on that page

i am trying to replace existing files with new one

no luck?

You can try changing the theme

see hellotutorial.com/how-to-change-wordpress-theme-from-database

that is the last place i'm aware of that might contain code that is modifying header info

i recommend keeping plugins disabled while troubleshooting

if i change theme

then i will lost my css and all changes that i had made in that template

i uploaded new wp-admin and wp-include and related files

and now i am not seen anything

blank white screen

Good to hear. Please post the details of what you did as an answer on your post and mark it as the accepted answer.