PHP: Caching generated images.

This is a quick tutorial on how to save and cache dynamically generated images in PHP. Personally, I love the results that you can get by using PHP’s GD and Image functions.

However, dynamically generating images on the fly can be pretty resource intensive. This is especially true if your website receives a sizable amount of traffic.

In this example, I will generate a simple image using PHP and then save that image as a file.

The idea here is that our script will generate the image on the first request, and then serve up the cached physical file for any requests that occur afterwards.

Saving a GD image using PHP.

Let’s jump straight into the code.

<?php

//The path and filename that we give to the image when we cache it.
$filepath = 'cache/test.jpg';

//Check to see if this image already exists.
if(is_readable($filepath)){
    //The file exists, so lets display it.
    header("Content-type: image/jpeg");
    readfile($filepath);
    exit;
}

//The path to our font file (Arial TrueType font)
$fontPath = 'arial.ttf';

//The width of our image in pixels.
$imageWidth = 500;

//The height of our image in pixels.
$imageHeight = 300;

//The quality of our image. 0 = worst. 100 = best.
$imageQuality = 100;

//Create a new color image.
$image = imagecreatetruecolor($imageWidth, $imageHeight);

//Create the color white so that we can use it in our image.
$white = imagecolorallocate($image, 255, 255, 255);

//Create the color black as well.
$black = imagecolorallocate($image, 0, 0, 0);

//Fill our image with a black background.
imagefill($image, 0, 0, $black);

//The text we're displaying on our image.
$stringToDisplay = 'Hello World';

//Add the text to our image.
imagettftext($image, 40, 0, 100, 100, $white, $fontPath, $stringToDisplay);

//Save the image to the cached filepath location.
imagejpeg($image, $filepath, $imageQuality);

//Display the image to the browser.
header("Content-type: image/jpeg");
imagejpeg($image, null, $imageQuality);

Below is a drill-down / explanation of the code above:

  1. We set the location of our cached file.
  2. We check to see if the cached file exists by using PHP’s is_readable function. This function checks to see if a given file exists AND if it is readable by the web server.
  3. If the given $filepath is readable, we presume that a cached version of our image exists. In this case, we display the image by setting the content type to “image/jpeg” and by using the function readfile. In case you didn’t already know – readfile simply “reads a file and then writes it to the output buffer.” We then use the exit function to halt further execution of the script.
  4. If the given $filepath is not readable, we generate the image using PHP’s GD and Image functions. As you can see, the image in question is pretty simple. We’re merely outputting a white “Hello World” message onto a black background.
  5. After we’ve generated our image, we save the image to our given $filepath by using the imagejpeg function. The second parameter of this function is extremely important. If the second parameter is a filepath, then the function will save the image. If this parameter is set to NULL, then it will output the image directly. In our case, we save the image and then we ALSO output the image directly.

Hopefully, you found this tutorial to be useful!

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