I've noticed that the first condition does not work

if (empty($ss)) {
  echo "please write your search words";
}

but the second does

else if ($num < 1)   {
   echo  "not found any like ";

full code

<?php
require_once "conf.php";
$ss= $_POST["ss"];
$sql2=("SELECT * FROM student WHERE snum = $ss");
$rs2 = mysql_query($sql2) or die(mysql_error());
$num = mysql_num_rows($rs2);

if (empty($ss)) {
    echo "please write your search words";
}
else if ($num < 1 ) {
    echo  "not found any like ";
}else {
    $sql=("SELECT * FROM student WHERE snum = $ss ");
    $rs = mysql_query($sql) or die(mysql_error());

    while($data=mysql_fetch_array($rs)) {
?>
<div id="name">
    <table align="center"  border="3" bgcolor="#FF6666">
        <tr>
            <td><? echo $data ["sname"]." "."????? ??????"; ?></td>
        </tr>
    </table>
</div>
<div id="ahmed">
    <table width="50%" height="50" align="center" border="2px" bgcolor="#BCD5F8">
        <tr>
            <td width="18%"><strong>???????</strong></td>
            <td width="13%"><strong>?????</strong></td>
            <td width="13%"><strong>?????</strong></td>
            <td width="14%"><strong>????</strong></td>
            <td width="12%"><strong>????</strong></td>
            <td width="30%"><strong>??????</strong></td>
        </tr>
        <tr>
            <td>100</td>
            <td>100</td>
            <td>100</td>
            <td>100</td>
            <td>100</td>
            <td><strong>?????? ????????</strong></td>
        </tr>
        <td><? echo $data['geo']; ?></td>
        <td><? echo $data['snum']; ?></td>
        <td><? echo $data['math']; ?></td>
        <td><? echo $data['arab']; ?></td>
        <td><? echo $data['history']; ?></td>
        <td><strong>????? ??????</strong></td>
        </tr>
        <tr>
            <td colspan="5" align="center" valign="middle">
                        <? $sum= $data['geo'] + $data['snum'] +
                            $data['math'] + $data['arab'] +
                            $data['history'];
                        echo $sum ;
                        ?>
            </td>
            <td><strong>????? ???????</strong></td>
        </tr>
        <tr>
            <td colspan="5" align="center">
                        <?
                        $all=500 ;
                        $sum= $data['geo'] + $data['snum'] +
                            $data['math'] + $data['arab'] +
                            $data['history'];
                        $av=$sum/$all*100 ;
                        echo $av."%" ;
                        ?>
            </td>
            <td><strong>
                    ?????? ???????
                </strong></td>
        </tr>
    </table>

</tr>
</div>
        <?
    }
};

the full code link is

http://www.mediafire.com/?2d4yzdjiym0