<?php
$existance = mysql_query("SELECT username FROM users WHERE username = '" . $username . "'");
if( !$existance ){
    $query    = "INSERT into `users` (username, password, email, trn_date) VALUES ('$username', '".md5($password)."', '$email', '$trn_date')";
    $result   = mysql_query( $query );
    if ( $result ) {
        echo "<div class='form'><h3>You are registered successfully.</h3><br/>Click here to <a href='login.php'>Login</a></div>";
    }
    else{
        //unsuccessful insertion
    }
} else {
    //the user existed already, choose another username
}
?>

if ($num_rows > 1) { $error = 1; } else { $error = 0; }
if ($error == 0) {  $query    = "INSERT into `users` (username, password, email, trn_date) VALUES ('$username', '".md5($password)."', '$email', '$trn_date')";
$result   = mysql_query($query);
}