Código PHP:
<?php session_start();
$a = isset($_GET['a']) ? $_GET['a'] : null;
if (isset($_POST['entrar'])){
echo "ENTRO AAAaaaaaaa";
require("conectar.php");
$sql = mysqli_query($conn,"select * from usuario");
while($rs = mysqli_fetch_array($sql)){
if ($_POST['user']==$rs['name'] and $_POST['pass']==$rs['pass']){
$_SESSION["usuario"] = $_POST['user'];
echo "<script> location.href='inicio.php'; </script>";
}
else
{
$a="<font color=\"red\" size=\"2\">Nombre de usuario o contrasena incorrecta.</font>";
}
}
}
?>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Administrador</title>
<link rel="stylesheet" href="css/admin.css" type="text/css" media="screen">
</head>
<body>
<table width="100%" height="100%" border="0" cellspacing="0" cellpadding="0">
<tr>
<td align="center" valign="middle">
<form action="" method="post">
<table border="0" cellspacing="6" cellpadding="6">
<tr>
<td colspan="2" align="center"><img src="image/logo.png" width="102" height="85"></td>
</tr>
<tr>
<td colspan="2" align="center"><?php echo $a?> </td>
</tr>
<tr>
<td><img src="image/usuario.png" width="54" height="54"></td>
<td><label for="user"></label>
<input type="text" name="user" id="user" class="campo" tabindex="1"></td>
</tr>
<tr>
<td><img src="image/clave.png" width="54" height="54"></td>
<td><label for="pass"></label>
<input type="password" name="pass" id="pass" class="campo" tabindex="2"></td>
</tr>
<tr>
<td colspan="2" align="right"><img src="image/recuperar.png" width="98" height="10"></td>
</tr>
<tr>
<td colspan="2" align="center"><input type="image" value="entrar" name="entrar" src="image/login.png" /></td>
</tr>
</table></form></td>
</tr>
</table>
</body>
</html>