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<?php
if(isset($_POST['entra']) && $_POST['entra'] == 'entrando'){
    $username = $_POST['usuario'];
    $password = $_POST['senha'];
    if(empty($username) || empty($password)){
        $error = "Por favor digite seu login e senha ".$shortname.".";
    }else{
        $query = mysql_query("SELECT * FROM users WHERE username='$username' AND password='$password' LIMIT 1");
        if(mysql_num_rows($query) == "0"){
            $error = "Usuario ou senha ".$shortname." incorretos!";
        }else{
                if(mysql_num_rows($query) == "1"){
                $_SESSION['username'] = $username;
                $_SESSION['password'] = $password;
                echo '<script language= "JavaScript">location.href="principal.html"</script>';
                }else{
                    $error = "Usuario ou senha ".$shortname." incorretos!";
                }
        }//Else User incorreto
    }//Else Empty
}//IF
?>
<form method="post" action="" class="form">
<?php if(isset($error)){ ?>
<div class="alert alert-error">
<b>Erro: </b> <?php echo $error; ?>
</div>
<?php } ?>
<input type="text" name="usuario" class="input" placeholder='Nome de Usuario' required/>
<input type="password" name="senha" class="input" placeholder='Senha' style="margin-top:8px;" required/>
<input type="hidden" name="entra" value="entrando"><input type="submit" value="Login" class="submit verde"/>
</form>