[SOLVED] Nothing Appears

<?php
include "classes/recipes.php";
include "classes/render.php";
include "classes/recipecollection.php";
include "inc/recipes.php";

$cookbook = new RecipeCollection("Treehouse Recipes");
$cookbook->addRecipe($lemon_chicken);
$cookbook->addRecipe($granola_muffins);
$cookbook->addRecipe($belgian_waffles);
$cookbook->addRecipe($pepper_casserole);
$cookbook->addRecipe($lasagna);
$cookbook->addRecipe($dried_mushroom_ragout);
$cookbook->addRecipe($rabbit_catalan);
$cookbook->addRecipe($grilled_salmon_with_fennel);
$cookbook->addRecipe($pistachio_duck);
$cookbook->addRecipe($chili_pork);
$cookbook->addRecipe($crab_cakes);
$cookbook->addRecipe($beef_medallions);
$cookbook->addRecipe($silver_dollar_cakes);
$cookbook->addRecipe($french_toast);
$cookbook->addRecipe($corn_beef_hash);
$cookbook->addRecipe($granola);
$cookbook->addRecipe($spicy_omelette);
$cookbook->addRecipe($scones);

echo Render::listRecipes($cookbook->getRecipeTitles()); 

//echo Render::displayRecipe($belgian_waffles);

<?php

class Recipe 
{
    private $title;
    private $ingredients = array();
    private $instructions = array();
    private $yield;
    private $tag = array();
    private $source = "Alena Holligan";

    private  $measurements = array(
        "tsp",
        "tbsp",
        "cup",
        "oz",
        "lb",
        "fl oz",
        "pint",
        "quart",
        "gallon"
    );

    public function __construct($title = null)
    {
         $this->setTitle($title);
    }
    public function __toString()
    {
        $output = "You are calling a " . __CLASS__ . " object with the title \"";
        $output .= $this->getTitle() . "\"";
        $output .= "\nIt is stored in " . basename(__FILE__) . " at " . __DIR__ . ".";
        $output .= "\nThis display is from line " . __LINE__ . " in method " . __METHOD__;
        $output .= "\nThe following methods are available for objects of this class: \n";
        $output .= implode("\n", get_class_methods(__CLASS__));
        return $output;
    }
    public function setTitle($title)
    {
        if (empty($title)) {
          $this->title = null; 
        } else {
          $this->title= ucwords($title);
        }
    }

    public function getTitle()
    {
        return $this->title;
    }

    public function addIngredient($item, $amount = null, $measure = null)
    {
        if ($amount != null && !is_float($amount) && !is_int($amount)) {
            exit("The amount must be a float: " . gettype($amount) . " $amount given");
        }
        if ($measure != null && !in_array(strtolower($measure), $this->measurements)) {
            exit("PLeasue enter a valid measurement: " . implode(", ", $this->measurements));
        }
        $this->ingredients[] = array(
            "item" => ucwords($item),
            "amount" => $amount,
            "measure" => strtolower($measure)
        );

    }

    public function getIngredients()
    {
        return $this->ingredients;
    }

    public function addInstruction($string)
    {
        $this->instructions[] = $string; 
    }

    public function getInstructions()
    {
        return $this->instructions; 
    }

    public function addTag($tag)
    {
        $this->tags[] = strtolower($tag);
    }

    public function getTags()
    {
        return $this->tags;
    }

    public function setYield($yield)
    {
        $this->yield = $yield;
    }

    public function getYield()
    {
        return $this->yield;
    }

    public function setSource($source)
    {
        $this->source = ucwords($source);
    }

    public function getSource()
    {
        return $this->source;
    }


}

<?php

class Render
{
        public function __toString()
        {
            $output = "The following methods are available for " . __CLASS__ . "objects of this class: \n";
            $output .= implode("\n", get_class_methods(__CLASS__));
            return $output;
        }

        public static function listRecipes($titles)
        {
            asort($titles);
            return implode("\n", $titles);
        }

        public static function listIngredients($ingredients)
        {
            $output = "";
            foreach ($ingredients as $ing) {
                $output .= $ing["amount"] . " " . $ing["measure"] . " " . $ing["item"];
                $output .= "\n";
            }
            return $output;
        }

        public static function displayRecipe($recipe)
    {
        $output = "";
        $output .= $recipe->getTitle() . " by " . $recipe->getSource();
        $output .= "\n";
        $output .= implode(", ", $recipe->getTags());
        $output .= "\n\n";
        $output .= self::listIngredients($recipe->getIngredients());
        $output .= "\n";
        $output .= implode("\n", $recipe->getInstructions());
        $output .= "\n";
        $output .= $recipe->getYield();

        return $output;
    }  
}

<?php

class RecipeCollection
{
    private $title;
    private $recipe = array();

    public function __construct($title = null)
    {
         $this->setTitle($title);
    }

    public function setTitle($title)
    {
        if (empty($title)) {
          $this->title = null; 
        } else {
          $this->title= ucwords($title);
        }
    }

    public function getTitle()
    {
        return $this->title;
    }

    public function addRecipe($recipe)
    {
        $this->recipes[] = $recipe;
    }

    public function getRecipes()
    {
        return $this->recipes; 
    }

    public function getRecipeTitles()
    {
        $titles = array();
        foreach ($this->recipes as $recipe) {
          $title[] = $recipe->getTitle();
        }
        return $titles;
    }

}