session variables - PHP retrieving username of current user on page and then using it in a sql query -

i making website user has log in/create account , can type in terms search on twitter on own personal dashboard.

to make sure logged in on every page have code:

<?php session_start();  if (isset($_session['loggedin'])) { $_session["username"] = $username;     echo "(you logged in)"; } else {  echo "(not logged in)"; echo"<p><a href = 'login.php'>log in</a>"; echo"<p><a href = 'createaccount.php'>create account</a>";      exit(); } ?> 

in code try , take username logged in login page

    <?php  session_start();      //connect database $db = mysqli_connect("number", "name", "password"); //msg if not connected  if (!$db) { echo "sorry!i can't connect database"; } if(isset($_post['submit']))  {  $username =$_post['username']; $password = $_post['password'];  if(empty($username) or empty ($password)) {  echo "you missed something"; }  else {  if(!empty($username) && !empty($password)) {  mysqli_select_db ($db,"name");  $qry=" select username login username= '$username' , password = '$password';";  $result = mysqli_query($db,$qry); $num_rows = mysqli_num_rows($result);  if(($num_rows) == 1) {             //username successful             header("location:index.php"); $_session['loggedin'] = 'yes';     $username = $_session['username'];          }          else {             //username failed print '<script type="text/javascript">'; print 'alert("incorrect infomation. try again.")'; print '</script>';                       }   }   }  }  ?> 

and on dashboards enter terms want can't figure out how know logged in there (im trying term 1 atm)

<?php    session_start();   //connect database $db = mysqli_connect("number", "name", "password"); //msg if not connected  if (!$db) { echo "sorry!i can't connect database"; } $term1 =$_post['term1']; $term2 = $_post['term2']; $term3 = $_post['term3']; $_session["username"] = $username;  if(isset($_post['submit']))  {   mysqli_select_db ($db,"sarahpattison");  $query2 = "insert terms (term) values ('$term1') ;";  $result = mysqli_query($db,$query2); $term1id = mysqli_insert_id($db);  echo $term1id; if ($result){  $qry5="insert userterms(userid,termid) select login.userid, terms.termid login, terms login.username = '$username' , terms.termid='$term1id';"; $result8 = mysqli_query($db,$qry5); }    }    ?> 

the $username variable isn't working , im wondering shouldn't have checklogin part different page. don't know session variables.

try

$username = $_session["username"]; 

instead of

$_session["username"] = $username; 

in third file