The answer of your question is:
<?php
session_start();
session_destroy();
?>
<meta charset="UTF-8">
<html>
<form method="GET" action="datos.php">
<?php
if(isset($_GET['usuario'])){
if(strcmp("ana",$_GET['usuario'])==0){
if(strcmp("abc123.",$_GET['contrase?a'])==0){
$_SESSION['usuario']=$_GET['usuario'];
}
else{
echo"
<script>
alert ('Usuario o contrase?a incorrectos');
window.location='login.php';
</script>"
;}
}
if(strcmp("xan",$_GET['usuario'])==0){
if(strcmp("abc123.",$_GET['contrase?a'])==0){
$_SESSION['usuario']=$_GET['usuario'];
}
else{
echo"
<script>
alert ('Usuario o contrase?a incorrectos');
window.location='login.php';
</script>"
;}
}
}
if(isset($_SESSION['usuario'])){
$conexion=mysqli_connect("localhost","root","","folla14")or die(mysqli_error());
if($conexion){
mysqli_set_charset($conexion,"UTF8");
echo "
<input type='submit' value='Lista completa' name='listac'>
"
;}
}
else{
echo"
<script>
alert ('Usuario o contrase?a incorrectos');
window.location='login.php';
</script>";
}
?>