$id = 123;

if (empty($_POST['nick']) )
{
    echo '   <form method="post" >' ;
    echo ' <p> Vas nick  <br> ';
    echo ' <input type="text" name="nick">  ';
    echo ' </p> ';
    echo '
        <input type="hidden" name="odeslano" value="true">
        <input type="Submit" name="odeslat" value="Dat do aukce">
        </form>
     ';
}
else
{
    $kontrola=mysql_query("select count(*) from characters WHERE char_name='".$_POST['nick']."'");
    $kontrola2 =  mysql_fetch_row($kontrola);
    if ($kontrola2[0] < 1 )
    {
        echo "Tenhle nick nexistuje";
    }
    else
    {
        echo $kontrola2[0] ;
    }
.....
}