hoe met links omgaan in php?

<a href="" name="bla" onclick="">
			test
			</a>
			
			<?php
			
			if (isset($_POST['bla'])){
				include("bla.inc");
			}
			else{
				echo "oops foutje";
			}
			?>

<a href="" name="bla" onclick="">
			test
			</a>
 
			<?php
 
			if (isset($_POST['bla'])){
				include("bla.inc");
			}
			else{
				echo "oops foutje";
			}
			?>

<a href="?pag=hallo.php">Hallo</a>
<?php
include $_GET['pag'];
?>

<a href="?pag=hallo.php">Hallo</a>
<?php
include $_GET['pag'];
?>

<?php
if($_GET['page'] == 'forum')
{
include 'forum.php';
}
?>

<?php
if($_GET['page'] == 'forum')
{
include 'forum.php';
}
?>

<a href="?pag=home.php">Home</a>
<?php
if(empty($_GET['pag'])){
  include 'home.php';
}else{
  if(file_exists($_GET['pag'])){
    include $_GET['pag'];
  }else{
    include 'error.php';
  }
}
?>

<a href="?pag=home.php">Home</a>
<?php
if(empty($_GET['pag'])){
  include 'home.php';
}else{
  if(file_exists($_GET['pag'])){
    include $_GET['pag'];
  }else{
    include 'error.php';
  }
}
?>

<?php
$page ="";
?>

<a href="?page=test">testpagina</a>

<?php
if($_GET['page'] == "test")
	include("test.php");
?> 

<?php
$page ="";
?>
 
<a href="?page=test">testpagina</a>
 
<?php
if($_GET['page'] == "test")
	include("test.php");
?>

<h1>Een site</h1>
<div id='navigatie'>
   <a href="?page=test">Eerste Testpagina</a>
   <a href="?page=test2">Tweede Testpagina</a>
</div>
<div id='beschrijving'>
   <?php
   $paginas = Array("test","test2"); //bestaande pagina's (dit is nodig voor veiligheid

   if(in_array($_GET['page'],$paginas)) {
     include($_GET['page'].".php");
   } else {
     include("home.php"); // de specifieke layout van de index gaan halen. 
   }
   ?>
</div>

<h1>Een site</h1>
<div id='navigatie'>
   <a href="?page=test">Eerste Testpagina</a>
   <a href="?page=test2">Tweede Testpagina</a>
</div>
<div id='beschrijving'>
   <?php
   $paginas = Array("test","test2"); //bestaande pagina's (dit is nodig voor veiligheid
 
   if(in_array($_GET['page'],$paginas)) {
     include($_GET['page'].".php");
   } else {
     include("home.php"); // de specifieke layout van de index gaan halen. 
   }
   ?>
</div>