PHP Error, call to member function. Lost.

<?php

class Render {

            public function __toString() {
                   $output = "";
                   $output .= "<br />The following methods are available for " . __CLASS__ . "objects  <br />";
                   $output .= implode("<br />", get_class_methods(__CLASS__));

                return $output;
            }

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

            /*display Recipe method*/           
            public static function displayRecipes($recipe) {

                $output = "";
                $output .= "<br />---------<br />";
                $output .= $recipe->getTitle() . " by " . $recipe->getSource();
                $output .= "<br />";
                $output .= self::listIngredients($recipe->getIngredients() );
                $output .= implode(", ", $recipe->getTags());
                $output .= "<br />";
                $output .= "----Instructions----";
                $output .= "<br />";
                $output .= implode("<br />", $recipe->getInstruction() );
                $output .= "<br />";            
                $output .= "<br />";            
                $output .= $recipe->getYield();
                $output .= "<br />";                
                //$output .= $recipe->getSource();

                /*<br /> for new line when outputting to a html file.*/
                return $output;

            }


        public static function listRecipes($titles) {

           //sort the titles array;
           asort($titles);

           return implode("\n", $titles);
        }
}

?>

        <?php

        class Recipe {

            //These are properties of the Recipe class.  Some are arrays, some are strings.     
            private $title;
            private $ingredients = array();
            private $instructions = array();
            private $recYield;
            private $tag = array();
            private $source = "Jonathan";
            private $measurements = array(
                "tsp",
                "tbsp",
                "cup",
                "oz",
                "lb",
                "fl oz",
                "pint",
                "quart",
                "gallon",

            );

            /*getters and setters for private properties*/
            /*public function setTitle($title) {
                  $this->title = ucwords($title);
            }*/


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

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

            //"add" function for adding ingredients to cookbook.
            public function addIngredient($item, $amount = null, $measure = null) {


                // If there's a value of amount and it is neither a float or an integer, display a message. 
                if ($amount != null && !is_float($amount) && !is_int($amount)) {
                    exit("The amount must be a float: " . gettype($amount) . " $amount given");
                }
                // if there  is a value but not a valid measurement from measurement array display a message.
                if ($measure != null && !in_array(strtolower($measure), $this->measurements)) {
                    exit("Please enter a valid measurement: " . implode(", ", $this->measurements));
                }       
                //add ingredients as an associative array   "items" =>ucwords($item),

                $this->ingredients[] = array(
                    "item" => ucwords($item),
                    "amount" => $amount,
                    "measure" => strtolower($measure)
                );
            }

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

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

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

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

            }

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

            }

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

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

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

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

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

            }

            public function __toString() {
                   $output = "You are calling a " . __CLASS__ . " object with the title \"";
                   $output .= $this->getTitle() . "\"";
                   $output .= "<br />It is stored in " . basename(__FILE__) . " at " . __DIR__ . ".";
                   $output .= "<br />This display is from line " . __LINE__ . " in method " . __METHOD__;
                   $output .= "<br />The following methods are available for objects of this class: <br />";
                   $output .= implode("<br />", get_class_methods(__CLASS__));

                return $output;
            }

        }
        ?>

<html>
<head>
    <title>PHP: Cookbook using Object Oriented Programming</title>

    <link href="https://fonts.googleapis.com/css?family=Roboto+Slab" rel="stylesheet">
    <link href="styles.css" rel="stylesheet" type="text/css" />

</head>
<body>

    <header>
        <img class="logo" src = "logo.png" />
    </header>

    <div class="pp_head">
        <h1>PHP: Cookbook using Object Oriented Programming</h1>

        <p class="center"><a href="">Recipe List</a></p>
    </div>

    <div class = "content">
        <!-- -->

        <h1>Cookbook</h1>

        <?php

            include "recipe.php";
            include "render.php";
            include "inc/recipes.php";
            include "recipiecollection.php";

            echo "Cookbook: Recipies by Alena Holligan and Ken Alger";

            //echo Render::displayRecipe($recipe1);
            //echo Render::displayRecipe($recipe2);
            //echo Render::displayRecipe($lemon_chicken);
            //echo Render::listIngredients($ingredients);

            /***************************************
            Display Recipies as Collections Cookbook
            ***************************************/

            $cookbook = new RecipeCollection("Treehouse Recipies");

            echo $cookbook->addRecipe($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);

            //var_dump($cookbook);

            //cookbook.php  - to display the titles.
            echo Render::displayRecipes($cookbook->getRecipeTitles());
        ?>


    </div>

    <footer>
        <p>PHP: Cookbook using Object Oriented Programming. From Teehouse by Jonnie Grieve Digital Media</a></p> 
    </footer>
</body>

</html>