php - insert inside $_SESSION a string -

I have a problem where I have not found a solution on the Internet.

My peace code is:

  $ username = $ _POST ["nkn"]; $ _SESSION ['user'] = $ username;   
 Then, when I use it:  
  $ _SESSION ['user'] echo;   
 Nothing appears  
 There may be a problem on the second line of the first part of the code?  
 If yes, then I have not found any PHP method like  strcpy  or similar. How do I copy a string?   
 
 
  Reason:   
  session_start ( ); Using the sessions  should not be present at the top of the affected pages.  
  Solution:   
  session_start ();  Seasons must be placed at the top of each page, usually the PHP opening tag  & lt ;? Php .   
  Example:   
  & lt ;? Php session_start (); // The rest of the code? & Gt;   
 More information on this topic can be accessed on the PHP.net website.