PHP Callback-function array_walk

Hello i need help with this code, the problem is that i need to make a callback-function that only shows me the Price of the array
Like this image >> this is my code i tried much things but i couldn't figure out help me pls

    $boeken = array(

        array("titel"=> "Stoner", "auteur" => "John Williams",
        "genre"=> "Fictie", "prijs"=> 19.99),   
        array("titel"=> "De cirkel", "auteur" => "Dave Eggers",
        "genre"=> "Fictie", "prijs"=> 22.50),   
        array("titel"=> "Rayuela", "auteur" => "Cortazar",
        "genre"=> "Fictie", "prijs"=> 25.50)
      ); 

      function prijslijst(){
          array_walk($boeken, "prijslijst");
      }




    ?>```