pagination kvartal og år via GET url

PHP kode 

 <?php
 // GET QUARTER FUNCTION
 function get_kvt($month){
     $q = 0;
     switch ($month) {
         case '1':
         case '2':
         case '3':  
             $q = 1;
             break;
         case '4':
         case '5':
         case '6':
             $q = 2;
             break;
         case '7':
         case '8':
         case '9':
             $q = 3;
             break;
         case '10':
         case '11':
         case '12':
             $q = 4;
             break;
     }
 
     return $q;
 }
 
 // if the year is set in the URL then give it to me, else give me the real year
 $current_year = (isset($_GET['year'])? $_GET['year']: date("Y"));
 $next_year = $current_year + 1;
 $prev_year = $current_year -1;
 $first_year = 2010;
 $last_year = 2030;
 
 // if the quarter is set in the URL then give it to me, else give me the quarter from my function year
 $current_quarter = (isset($_GET['q'])? $_GET['q']: get_kvt(date("n")));
 $next_quarter = $current_quarter + 1;
 $prev_quarter = $current_quarter -1;
 $first_quarter = 1;
 $last_quarter = 4;
 
 
 // if prev quarter is less then 1 then change the quarter to 4
 if ($prev_quarter < $first_quarter) {
     $prev_quarter = $last_quarter;
     if ($first_quarter == 1) {
         $current_year = $next_year; 
     }
      
 }
 // if prev quarter is greater then 4 then change the quarter to 1
 if ($next_quarter > $last_quarter){
     $next_quarter = $first_quarter;
     if ($last_quarter == 4) {
         $current_year = $prev_year;
     }
    
 }
 
 
 
 // prevent exceed the first and last year
 if($current_year < $first_year) {
     $current_year = $first_year;
 }
 if($current_year > $last_year) {
     $current_year = $last_year;
 }
 // DEBUG PREVIEW
 echo $prev_quarter . " <--Prev Q |  Next Q --> " . $next_quarter . "<br>";
 echo $prev_year . " <--Prev Y | Next Y --> " . $next_year . "<br>";
 
 // NAVIGATION
 echo "
  <a href='nav.php?q=$prev_quarter&y=$current_year'>Previous</a>
  <a href='nav.php?q=$next_quarter&y=$current_year'>Next</a>
 
  ";
 
 
 // OUTPUT
 echo "<br>current year <h2>(" . $current_year . ")</h2>";
 echo "<h2>current quarter (" . $current_quarter . ")</h2>";
 
  ?>