How to use PHP's GD extension?

In web development, common operations such as making image thumbnails and watermarks are often required. It is very convenient to use PHP to perform these operations. PHP's GD extension is a good choice, it provides some convenient methods to process images.

1. Install GD extension

If your PHP does not have the GD extension installed, you need to install the extension first. In Linux, you can use the shell command sudo apt-get install php7.0-gd to install it. In Windows, you can remove the comment extension=gd2 in the PHP configuration file php.ini. That’s it.

2. Create a basic image

Now let’s create a simple PNG image. PHP's imagecreate() function can be used to create a basic image, and the width and height can be specified.

$width = 200;
$height = 200; 
$image = imagecreate($width, $height);

Here we create a 200×200 PNG image. For convenience, we save it locally:

header("Content-type: image/png"); 
imagepng($image, "test.png");

After this image is generated, it can be found in test.png. This step can be used to create basic images for many needs.

3. Set text

We often need to write some text on pictures. The GD extension provides the imagestring() function to complete this operation:

$bgColor = imagecolorallocate($image, 255, 255, 255);
$textColor = imagecolorallocate($image, 0, 0, 0);
$fontFile = 'arial.ttf';
$fontSize = 20;
$text = 'Hello World!';
$x = 50;
$y = 150;

imagettftext($image, $fontSize, 0, $x, $y, $textColor, $fontFile, $text);

Here we set the background color, text color, font file, font size, text content and position, and use the imagettftext() function to render the text onto the image. To use the imagettftext() function, you need to ensure that the FreeType library has been correctly installed on the server.

4. Rotate the image

In addition to writing text on the image, we also need to rotate the image from time to time. The GD extension also provides the imagecopyrotate() function to complete this operation:

$degrees = -45; 
$newImage = imagerotate($image, $degrees, 0);
imagepng($newImage, 'rotated.png');

In this example, the original image will be rotated 45 degrees, and finally a new PNG file "rotated.png" will be generated. In addition to rotation, there are many other image processing operations that can be completed using GD extensions.

5. Add watermark

Finally, let’s add a watermark to the image. This can be done using the imagecopy() function. We can first reduce the watermark and set the opacity:

$watermark = imagecreatefrompng('watermark.png');
$watermarkWidth = imagesx($watermark);
$watermarkHeight = imagesy($watermark);
$newWidth = $width / 2;
$newHeight = $height / 2;
$newWatermark = imagecreatetruecolor($newWidth, $newHeight);
imagecopyresampled($newWatermark, $watermark, 0, 0, 0, 0, $newWidth, $newHeight, $watermarkWidth, $watermarkHeight); 
imagecopymerge($image, $newWatermark, 0, 0, 0, 0, $newWidth, $newHeight, 50);

Here we create a watermark image and reduce its size by half, then add it to the original image through imagecopymerge() and set it 50% opacity.

In this way, we have learned some basic usage of GD extension. Of course, the GD extension has many other functions. If you need it, you can go to the official documentation to see more usage.

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