Introduction to Gemini's Image Generation and Symfony

Welcome to the first lesson of our course, "Creating Images with Gemini's Image Generation and Symfony." In this course, you will explore AI-driven image generation using Google's Gemini API and its native image generation model (Nano Banana, gemini-3.1-flash-image). Our journey begins with understanding how to set up the environment and generate a simple image using PHP with Symfony. This foundational lesson will set the stage for more advanced topics in subsequent units.

Setting Up the Environment

Before we dive into generating images, it's crucial to set up our environment correctly. First, ensure you have access to the Gemini API by retrieving your API key. This key is essential for authenticating your requests to the API. For this lesson, you'll need to use Composer, PHP's package manager, to install the necessary libraries. We'll be using Symfony components, particularly the Symfony HTTP Client for making API requests. Here's how to set up a basic Symfony project with the required components: Shell # Create a new Symfony project (or use an existing one) composer create-project symfony/skeleton gemini-image-app # Navigate to the project directory cd gemini-image-app # Install the HTTP Client component composer require symfony/http-client Note that the environment will be fully set up for the practices section, so you can focus on the exercises without worrying about the setup.

Getting the URL and API Keys

You can get more details about how to use the Gemini Image Generation API from Google's documents . There, you can get the important details about connecting to the API, namely the GEMINI_BASE_DIR and the GEMINI_API_KEY . For example, when connecting directly to Gemini, the GEMINI_BASE_URL will be https://generativelanguage.googleapis.com/ and the GEMINI_API_KEY is your own secret. In all of our examples and practices, we will provide these variables to you so you don't need to be concerned with them for now.

Using Libraries

For interacting with the Gemini API in PHP, we'll use Symfony's HTTP Client component. This provides a powerful and flexible way to make HTTP requests to external APIs. Since the Gemini API for image generation requires direct REST API calls, we'll use the Symfony HTTP Client to construct and send our requests. Later in the course, we'll encapsulate this functionality into reusable services.

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PHP

// Load API key from environment
$apiKey = $_ENV['GEMINI_API_KEY'] ?? null;
$baseUrl = $_ENV['GEMINI_BASE_URL'] ?? null;

if (!$apiKey || !$baseUrl) {
    throw new \Exception('❌ GEMINI_API_KEY or GEMINI_BASE_URL environment variable not set.');
}

// Define endpoint and payload
$endpoint = sprintf("%sv1beta/models/gemini-3.1-flash-image:generateContent?key=%s", $baseUrl, $apiKey);

JSON

{
  "contents": [
    {"parts": [{"text": "A monkey giving a penguin a high five"}]}
  ],
  "generationConfig": {
    "responseModalities": ["TEXT", "IMAGE"]
  }
}

PHP

use Symfony\Component\HttpClient\HttpClient;

$prompt = "A monkey giving a penguin a high five";

// Create request payload
$requestData = [
    'contents' => [
        [
            'parts' => [
                ['text' => $prompt],
            ],
        ],
    ],
    'generationConfig' => [
        'responseModalities' => ['TEXT', 'IMAGE'],
    ],
];

// Create HTTP client and make the request
$httpClient = HttpClient::create();
$response = $httpClient->request('POST', $endpoint, [
    'json' => $requestData,
]);

// Check if the request was successful
if ($response->getStatusCode() !== 200) {
    throw new \Exception(sprintf(
        '❌ API request failed. Status code: %d, Response: %s',
        $response->getStatusCode(),
        $response->getContent(false)
    ));
}

prompt
  └─> JSON request (contents + generationConfig)
        └─> Gemini API
              └─> JSON response (candidates[].content.parts[])
                    └─> part.inlineData.data  (Base64 string)
                          └─> base64_decode() -> saved image file

PHP

// Get the response content
$responseContent = $response->getContent();

// Parse response JSON
$responseData = json_decode($responseContent, true);

if (json_last_error() !== JSON_ERROR_NONE) {
    throw new \Exception(sprintf('❌ Failed to decode JSON response: %s', json_last_error_msg()));
}

PHP

// Create output directory in the public folder
$outputDir = 'public/images/';
if (!is_dir($outputDir)) {
    mkdir($outputDir, 0777, true);
}

// Save image if available
if (isset($responseData['candidates'][0]['content']['parts'])) {
    // A single response can carry more than one image part, so number the files
    // to make sure a later image never overwrites an earlier one.
    $imageCount = 0;

    foreach ($responseData['candidates'][0]['content']['parts'] as $part) {
        if (!isset($part['inlineData']['data'])) {
            continue;
        }

        $base64Img = $part['inlineData']['data'];
        $imageData = base64_decode($base64Img);
        $imageCount++;
        $outputPath = $outputDir . sprintf('output-image-%d.png', $imageCount);

        if (file_put_contents($outputPath, $imageData) === false) {
            throw new \Exception(sprintf('❌ Failed to save image to %s', $outputPath));
        }

        echo sprintf("✅ Image saved as %s\n", $outputPath);
    }
}

Introduction to Gemini's Image Generation and Symfony

Setting Up the Environment

Getting the URL and API Keys

Using Libraries

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Retrieving Your URL and API Keys

Constructing a Request

Processing the Response

Decoding the Image Data and Saving It to a File

Summary and Next Steps