Redirect to same path on new domain

Something like this should work:

$uri = $_SERVER['REQUEST_URI'];
Header( "HTTP/1.1 301 Moved Permanently" );
Header( "Location: http://newsite.com$uri" ); 

But if you can modify your web server's configuration instead, that would be a better place to do it.

You shouldn't do this in PHP. These things can be easily done in your .htaccess:

#Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_HOST} ^www.olddomain.com$[OR]
RewriteCond %{HTTP_HOST} ^olddomain.com$
RewriteRule ^(.*)$ http://www.newdomain.com/$1 [R=301,L]

This code will redirect olddomain.com/page.php to newdomain.com/page.php

It will also redirect folders: olddomain.com/folder/ to newdomain.com/folder/

By using this code google will also understand that you are switching domains and won't lower your page ranks for double content.

I will give you 2 functions which could be useful for some other thing;

function currentURL() {
     $pageURL = 'http';
     ($_SERVER["SERVER_PORT"] === 443) ? $pageURL .= "s" : '';
     $pageURL .= "://";
     if ($_SERVER["SERVER_PORT"] != "80") {
      $pageURL .= $_SERVER["SERVER_NAME"].":".$_SERVER["SERVER_PORT"].$_SERVER["REQUEST_URI"];
     } else {
      $pageURL .= $_SERVER["SERVER_NAME"].$_SERVER["REQUEST_URI"];
     }
     return $pageURL;
 }


function redirect2NewDomain () {
    $url = currentURL();
    if(filter_var($url, FILTER_VALIDATE_URL, FILTER_FLAG_HOST_REQUIRED) === FALSE) {
       return false;
    }
    # Get the url parts
    $parts = parse_url($url);
    Header( "Location : {$parts['scheme']}://{$parts['host']}" );
}

Ofcourse using .htaccess is much more easier and will be better for SEO;

RewriteEngine on
RewriteRule (.*) http://www.newdomain.com/$1 [R=301,L] 

I hope this helps