<form name="form1" method="get" action="">
  <input type="text" name="tahmin" id="tahmin">
  <input type="hidden" name="sayi" id="sayi" value="<?php
    if(!isset($_GET['sayi']))
  {
    $sayi=rand(1,100);
  echo $sayi;
  }
 else
  echo $_GET['sayi'];
  ?>">
  <input type="submit" name="gonder" id="gonder" value="Tahmin Et">
  
</form>
<?php
if(isset($_GET['tahmin']))
 $tahmin=$_GET['tahmin'];
if(isset($_GET['sayi']))
 $sayi=$_GET['sayi'];

if(isset($_GET['sayi']) && isset($_GET['tahmin']))
{
if($sayi==$tahmin)
 echo "Bildin";
elseif($sayi>$tahmin)
 echo "Yukarı";
elseif($sayi<$tahmin)
 echo "Aşağı";
}
?>