Lo siento se me olvido poner el codigo: index.php
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Sabias que?¿</title>
<link rel="stylesheet" href="estilo.css" type="text/css" media="screen" />
</head>
<body BGCOLOR="#7c9044">
<?php include('header.php'); ?>
<?php include('conexion.php'); ?>
<div style="float:left; width: 23%;">
<?php include('categorias.php'); ?>
<?php include('administrador.php'); ?>
</div>
<div style="float:left; width: 54%;">
<?php include('articulos.php'); ?>
</div>
<div style="float:right; width: 23%;">
<?php include('publicidad.php'); ?>
<center>
<img src="imagenes/cosplay-jade-mileena-mortal-kombat.jpg" width="194" height="271">
<br><br>
<img src="imagenes/Hannah Minx .jpg" width="194" height="290"></center>
<blockquote><a href="http://www.info.com/">Informacion</a></blockquote>
</div>
</body>
</html>
ahora el header.php
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<div style="float:left; width: 20%;">
<center>
<form action="" method="post" class="formularioBuscador"><br><br>
<input type="text" name="IdUsuario" placeholder="Palabras" required>
<input type="submit" name="submit" value="Buscar" class="button" role="button">
</form>
</center>
</div>
<div id="div_header" style="float:left; width: 60%;">
<center><?php echo "<a href='index.php'>";?><img src="imagenes/BDE-cabecera-1-png.png" width="650" height="150"><?php echo"</a>";?></center>
</div>
<div style="float:left; width: 20%;">
<form action="" method="post" class="formularioLogeo"><br><br>
<input type="text" name="IdUsuario" placeholder="Usuario" required><br><br>
<input type="password" name="PassUsuario" placeholder="Contraseña" required><br><br>
<input type="submit" name="submit" value="Ingresar" class="button" role="button">
</form>
</div>
ahora ctegorias.php
<h1>Categorias</h1>
<?php
$sqlQueryCat = mysql_query("SELECT * FROM sn_categorias", $db_link)or die(mysql_error);
echo "<ul>";
echo "<li><a href='index.php'>Todas</a>";
while($rowCat = mysql_fetch_array($sqlQueryCat))
{
echo "<li><a href='?categoria=$rowCat[catCategoria]&id=$rowCat[cat_ID]'>$rowCat[catCategoria]</a></li>";
}
echo "</ul>";
?>