sql - PHP choose another username -

i have made registration php file runs through authentication , connects database made in phpmyadmin. problem is, can put in same username without consequence , adds database, put; dogs username , again put same. how can make user told; username exists choose one. here's php far; please tell me insert it.

<?php  require('db.php');  // if form submitted, insert values database.  if (isset($_post['username'])) {      $username = $_post['username'];      $email    = $_post['email'];      $password = $_post['password'];      $username = stripslashes($username);      $username = mysql_real_escape_string($username);      $email    = stripslashes($email);      $email    = mysql_real_escape_string($email);      $password = stripslashes($password);      $password = mysql_real_escape_string($password);      $trn_date = date("y-m-d h:i:s");      $query    = "insert `users` (username, password, email, trn_date) values ('$username', '".md5($password)."', '$email', '$trn_date')";      $result   = mysql_query($query);      if ($result) {          echo "<div class='form'><h3>you registered successfully.</h3><br/>click here <a href='login.php'>login</a></div>";      }  } else {  ?>

you should query database before inserting record (user) users table. try code below:

<?php $username = mysql_real_escape_string( $username ); //sql injection prevention $existance = mysql_query("select username users username = '" . $username . "'"); if( !$existance ){     $query    = "insert `users` (username, password, email, trn_date) values ('$username', '".md5($password)."', '$email', '$trn_date')";     $result   = mysql_query( $query );     if ( $result ) {         echo "<div class='form'><h3>you registered successfully.</h3><br/>click here <a href='login.php'>login</a></div>";     }     else{         //unsuccessful insertion     } } else {     //the user existed already, choose username } ?>