php - I have trouble with my SESSION that change when i insert somethnig with POST -

i have trouble code below, put $level in $_session , don't know how that. $level replace user $_session['level'] , logged out.

<?php session_start();  if( $_post['action'] == 'add' ) {      $level = $_post['level'];  ?>  <form action="<?php $_server[php_self]; ?>" method="post" >      <input type="text" name="level" value="" placeholder="level" /><br />      <input type="hidden" name="action" value="add" />     <input type="submit" value="add user" /> </form> 

i'd common issue session variable forgetting session_start() in top of document. have put session_start() belongs, but have forgotten define session variable :)

this how it's done.

$_session['level'] = $_post['level'];