Código PHP:
echo "<select name=\"categorias[]\" multiple>\n".
$consultac=mysql_query("Select id,nombre from ego_shop_categorias");
while($consultarc=mysql_fetch_array($consultac))
{
echo "<option value=\"".$consultarc["id"]."\" ".($consultarc["id"] == $cat_p["id_categoria"] ? 'selected' : '').">".$consultarc["nombre"]."</option>\n";
}
echo "</select>\n".