UPDATE query doesn't work at a PHP form

[Sibusy]

Hello,

I'm trying to perform UPDATE with a PHP form , but the UPDATE query doesn't work 
anyway, returning"Can't perform the update", according to code below. It doesn't 
return any error at PHP nor at Mysql, I have tested the variables and they are beeing 
sent properly.Could someone help me???
Here goes the code : <?php 
    $db=mysql_connect("$host","$user","$senha") or die("Erro de Conexăo");

    mysql_select_db($dbnome,$db);

    $sql="UPDATE [LOW PRIORITY] grupoadministracao WHERE idgrupo = '$nid' SET nome = 
'$nome', resp= '$resp',      
             cnpj='$cnpj', cpf='$cpf', ie='$ie', 
endereco='$endereco',numero='$numero',cpto='$cpto', cep='$cep',  
             
telefone1='$telefone1',telefone2='$telefone2',fax='$fax',celular='$celular',email='$email',login='$login',senha='$senha'";
 

    $result=mysql_query($sql,$db);    
   
             if($result) {
                     echo ("Data<b>$nome</b>, registro nș<b>$nid</b> was successfully 
updated.<br>");
                     } else {
                            echo "Can't perform the update. ";
          mysql_error();
         }    


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