I can't open my index.php page after insert php code

Hello guys,
Does anyone can help me?
I've just made my simple index.php without any code, but after insert session code to check if any user is authenticated, my index.php doesn't work anymore.
Any fresh eyes could help me to see what and where the code is wrong?

<?
                    if ($_POST['entrar']=='Entrar!')
                       {
                        @session_start();
                        $result = mysql_query('select log_nome,senha from usuarios where log_nome="'.trim($nome).'";',$conexao);
                        if (mysql_num_rows($result)>0)
                        {
                         $usuSenha = mysql_result($result,0,senha);
                        else
                        {
                         $usuSenha = 'notuser';
                        }
                         if ($result and ($senha==$usuSenha) and ($usuSenha<>'notuser'))
                         {
                          @session_register($nome);
                          echo '<p align="center">';
                          echo '<font style="Tahoma" size=4>';
                          echo $nome.'</font><BR>';
                          echo '<font style="Tahoma" size=2>';
                          echo 'Voc� est� on-line</font>';
                          echo '</p>';
                         }
                         else
                         {
                          echo 'Usu�rio n�o autorizado';
                         }
                        }
                        else
                        {
                         echo '<form method="post" name="logar" action="">
                         <font style="Tahoma" size=1>
                               Nome: <input type="text" name="nome" value="" size="15"><BR>
                               Senha: <input type="text" name="senha" value="" size="15"><BR>
                         </font>
                         <p align="center">
                         <input onmouseover=this.style.cursor="hand" type="submit"
                         name="entrar" value="Entrar!" STYLE="border:0; height:18;
                         width=50; background:#E8F3FD">
                         </p>
                               </form>';
                        }
                    ?>

I'll apreciate any kind of help.
Thanks

Where is the file that sets this variable?

  if ($_POST['entrar']=='Entrar!')

What is 'Entrar!' ?

Maybe you want something like?

  if ($_POST['entrar'] !='Entrar')

?

Neo,
Thanks for your answer. I guess I�ve forgot to put the entire code.
Well, maybe now, with the entire index.php code it'll become easier to understand.

<? include "base.php"; ?>
<html>
<head>
<title>Example</title>
</head>
<body>
  <table border="0" width="100%" colspacing = 0>
  <tr>
  <td width = 150>
  <IFRAME align="center" name=banner1 frameborder=0 src="banner1.php" scrolling=no height="110" width="100%">
  Your browser doesn�t support frames
  </IFRAME>
  </td>
   <td bgcolor = "#FFFFC0" align = "center">
   <font style = "Tahoma" color="#0000FF" size = 6>
   Example
   </font>
   </td>
    <td width = 150>
    <IFRAME align="center" name=banner2 frameborder=0 src="banner2.php" scrolling=no height="110" width="100%">
    Your browser doesn�t support frames
    </IFRAME>
    </td>
    </tr>
     <tr>
     <td valign = "top">
     <table border="0" width="100%">
      <tr>
      <td align = "center" bgcolor = "#C0C0FF">
      <font style = "Tahoma" size = 2>Login
      </font>
      </td>
      </tr>
      <tr>
       <td bgcolor = "#D0D0D0">
       <?
       if ($_POST['entrar']=='Entrar!')
       {
        @session_start();
        $result = mysql_query('select log_nome,senha from usuarios where log_nome="'.trim($nome).'";',$conexao);
        if (mysql_num_rows($result)>0)
        {
         $usuSenha = mysql_result($result,0,senha);
        else
        {
         $usuSenha = 'notuser';
        }
         if ($result and ($senha==$usuSenha) and ($usuSenha<>'notuser'))
         {
          @session_register($nome);
          echo '<p align="center">';
          echo '<font style="Tahoma" size=4>';
          echo $nome.'</font><BR>';
          echo '<font style="Tahoma" size=2>';
          echo 'You are on-line</font>';
          echo '</p>';
         }
         else
         {
          echo 'User not authorized';
         }
        }
        else
        {
         echo '<form method="post" name="logar" action=""><font style="Tahoma" size=1>
         Name: <input type="text" name="nome" value="" size="15"><BR>
         Password: <input type="text" name="senha" value="" size="15"><BR>
         </font>
         <p align="center">
         <input onmouseover=this.style.cursor="hand" type="submit" name="entrar" value="Entrar!" STYLE="border:0; height:18; width=50; background:#E8F3FD">
          </p>
          </form>';
         }
         ?>
        </td>
        </tr>
        </table>
        <table border="0" width="100%">
        <tr>
         <td align="center" bgcolor="#C0FFFF">
         <font style="Tahoma" size=2>
         About us
         </font>
         </td>
         </tr>
          <tr>
          <td align="center" bgcolor="#C0FFFF">
          <font style="Tahoma" size=2>
          Talk to us
          </font>
          </td>
          </tr>
           <tr>
           <td align="center" bgcolor="#C0FFFF">
           <font style="Tahoma" size=2>
           Images
           </font>
           </td>
           </tr>
           </table>
           </td>
           <td>
           <IFRAME align="center" name=Display src="quemsomos.php" frameborder=0 scrolling=1 height="280" width="100%">
           Your browser doesn�t support frames
           </IFRAME>
           </td>
           <td valign = "top">
           <table border="0" width="100%">
           <tr>
            <td align="center" bgcolor="#C0C0FF">
            <font style="Tahoma" size=4>
            News
            </font>
            </td>
            </tr>
           <?
           $result = mysql_query('select titulo,conteudo from noticias;',$conexao);
           $quantos = mysql_num_rows($result);
           $i=0;
           for ($i==0; $i<$quantos; $i++)
           {
            echo '<tr><td bgcolor="#C0FFFF">';
            echo '<span style="font-size:8.0pt; font-family:Verdana; color:#000080">';
            echo '<a target= "Display" href="conteudo.php?texto='.mysql_result($result,$i,conteudo).'">';
            echo mysql_result($result,$i,titulo);
            echo '</a></td></tr>';
           }
           ?>
           <tr>
            <td align="center" bgcolor="#C0FFFF">
            <font style="Tahoma" size=2>
            Weather
            </font>
            </td>
            </tr>
             <tr>
             <td align="center" bgcolor="#FFFFDD">
             <font style="Tahoma" size=2>
             Test test test test <BR>
             test test test test <BR>
             test test test test
             </font>
             </td>
            </tr>
           </table>
          </td>
         </tr>
        </table>
      </body>
</html>

PS.: "Entrar!" is the action to perform login. You can consider login=entrar
The exclamation mark before the "=" works?

Your first line has a syntax error.

Do you see it?

Here?

<? include "base.php"; ?> 

But I have this include in my other pages and they work fine.
Now, I don�t know where in the 3 php scripts in my index.php are the mistakes.
My base.php code is this:

<?
  $conexao = mysql_connect('localhost','root','root') or die (mysql_error());
  mysql_select_db("criandosites");
?>

What are the errors in the web server log files when you run this?

I can�t believe it !!! I�ve just found my stupid mistake!!

<?
       if ($_POST['entrar']=='Entrar!')
       {
        @session_start();
        $result = mysql_query('select log_nome,senha from usuarios where log_nome="'.trim($nome).'";',$conexao);
        if (mysql_num_rows($result)>0)
        {
         $usuSenha = mysql_result($result,0,senha);
        else
        {
         $usuSenha = 'notuser';
        }
...

Now, look what I�ve forgot to insert or better... finnish !!

<?
       if ($_POST['entrar']=='Entrar!')
       {
        @session_start();
        $result = mysql_query('select log_nome,senha from usuarios where log_nome="'.trim($nome).'";',$conexao);
        if (mysql_num_rows($result)>0)
        {
         $usuSenha = mysql_result($result,0,senha);
        }
        else
        {
         $usuSenha = 'notuser';
        }
...

I forgot to finnish the code ( { } ).
Anyway, thanks for your patience Neo.
regards.