Nothing happens when I run cookbook.php

<?php

class Recipe
{
    private $title;
    private $ingredients = array();
    private $instructions = array();
    private $yield;
    private $tag = array();
    private $source = 'David Evans';

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

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

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

    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('Please 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

include 'classes/recipes.php';
include 'classes/render.php';

$recipe1 = new Recipe('My First Recipe');
$recipe1->setSource('Grandma Evans');
$recipe1->addIngredient('egg', 1);
$recipe1->addIngredient('flour', 2, 'cup');

$recipe2 = new Recipe('my second recipe');
$recipe2->setSource('Betty Crocker');

$recipe1->addInstruction('This is my first instruction');
$recipe1->addInstruction('This is my second instruction');

$recipe1->addTag('Breakfast');
$recipe1->addTag('Main Course');

$recipe1->setYield('6 servings');

echo $recipe1;
echo Render::displayRecipe($recipe1);

<?php

class Render
{
    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;
    }
}