Paste code

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html >
<head>
<meta http-equiv="Content-Type" c charset=utf-8" />
<title>Alterar</title>
</head>

<body>
<?php
   include('conexao.php');
 $mat = $_GET['matricula'];
 $sql = "select matricula, nome, datanasc from aluno where matricula='$mat'";
 $resultado = mysql_query($sql) or die ("Não foi possível realizar a consulta ao banco de dados");
 $linha = mysql_fetch_array($resultado);


?>
<form id="form1" name="form1" method="post" acti>
  <table width="326" border="1">
    <tr>
      <td width="163">Matricula</td>
      <td width="156"><label for="textfield"></label>
      <input type="text" name="textfield" id="textfield" value="<?php echo $linha['matricula'] ?>" /></td>
    </tr>
    <tr>
      <td>Nome</td>
      <td><label for="textfield2"></label>
      <input type="text" name="textfield2" id="textfield2" value="<?php echo $linha['nome'] ?>" /></td>
    </tr>
    <tr>
      <td>Data de nascimento</td>
      <td><label for="textfield3"></label>
      <input type="text" name="textfield3" id="textfield3" value="<?php echo $linha['datanasc'] ?>" /></td>
    </tr>
    <tr>
      <td> </td>
      <td> </td>
    </tr>
    <tr>
      <td> </td>
      <td> </td>
    </tr>
  </table>
  <input type="submit" name="button" id="button" value="Alterar"
</form>
</body>
</html>