Basically there are two parts to this How-To:
- Setting up a simple Twitter bot with PHP
- How to generate images with PHP

How can this even be done?

How is it possible to make something create images and tweet automatically? Wizardry??

I made this Twitter bot almost entirely with PHP - images and all. It tweets at certain times with a Cron job, and the database is build in MySQL. How the skins are organized is another story, haha.
The amount of code I used to do this was tiny. 50 lines for the images, 25 lines for making the tweet happen.

Setting up a simple Twitter bot with PHP

6 steps!

Step 1:
Selected the Twitter account you would like to use as a bot, and make sure you have a phone number verified on your account. Once you have that, log in to the Twitter app dashboard here: https://apps.twitter.com/. Click the Create New App button.

Step 2:
Fill in the required forms and accept the agreement. Click on the button "Create your Twitter application".

Step 3:
You're in! Go to the "Permissions" tab to allow your bot to have "Read and Write" permissions (so you can Tweet!).
Go to the "Keys and Access Tokens" tab and (re)generate your own "Access Token and App Secret" so you can tweet to your account. Here's what mine looks like. You will eventually need the API Key and Secret, and the Access Token and Secret, in order to use your bot.

Step 4:
Go to https://github.com/dg/twitter-php to get a simple API for the bot. Just copy the two files OAuth.php and twitter.class.php somewhere in the scope of your bot's main PHP file. They do all the work! ;)

Step 5:
Remember all those Keys, Tokens, and Secrets? Time to use them! Here's all the code you need to post to Twitter:

// Change PATH-TO to fit your organization
require_once 'PATH-TO/twitter.class.php';

// Change all 4 of these keys to the ones found in your app's "Keys and Access Tokens" tab.
$twitter = new Twitter("8DF4ndg0aaEjfajfjdajf994o1337lO1", "HjhljHhGhggk6f6DFtf9024sFCTf89ukNMnNcfX3l41S9a1",
	"9104893844-Tr0L0lo1LnENFLueuf81fNgFGCxmdJDiND8NIDd9",
	"flFfnFJenflkWENFf8NFjenfWNF2nelfNVNZPyuYKYsDZdT89n");
try {
	// Fix the image link in your own app
	$tweet = $twitter->send("I'm testing something cool, thanks to Zanz!", "OPTIONAL-IMAGE.png");
} catch (TwitterException $e) {
	// Error messages help you debug!
	echo("Couldn't tweet that. :( Reason: ".$e->getMessage());
}

That's about it! Simple, right? :D

Step 6:
Using a Cron Job to send the tweet can pretty useful! That way it's completely automatic! Remember that Twitter doesn't tolerate spam. I recommend that you don't go too crazy, or your Twitter account will be suspended.
Here's 3 examples of how you can set up your Cron Job with curl:

curl https://LINK-TO/TWITTER-BOT.php
curl https://LINK-TO/TWITTER-BOT.php?secretGETcode=someSecureKey
curl https://LINK-TO/TWITTER-BOT.php?secretGETcode=someSecureKey >/dev/null 2>&1

I went ahead and added extra security in the 2nd and 3rd lines. If the secret code is not recognized by the PHP file, it doesn't send. Permissions should be good enough though, but why not go the extra mile for safety?
The last line stops it from sending me e-mails each time it runs.

How to generate images with PHP

Without Image Magick, that is.

PHP's GD library can be a pain, but it can also be a huge help! To make the images in my bot's tweets, I split it up into 4 steps - drawing a generic background, loading and drawing the skin image, drawing text, and saving. I'll describe how I did these steps.

Be sure to read and explore the GD library yourself, if you get stuck with your own project.

Step 0:
Step 0 is just about how you go about testing your images. Basically, use this line of code at the top of your program to tell the browser to draw the file as an image:

header('Content-type: image/png');

Then at the end of the code, display the contents of $image in the browser with this line of code:

imagepng($image);
imagedestroy($image); //Then get rid of it in memory :D

Step 1:
The background image:

$image = imagecreatefrompng("BG-IMAGE.png"); // Load background
list($width, $height) = getimagesize("BG-IMAGE.png"); // Get the width and height

Step 2:
Loading and drawing the skin file was simple. I had the most trouble removing the backgrounds from skins, since they could either be transparent or not - in which case the background is automatically removed. You probably wont run into that issue.
Here's how to draw an image onto the background:

$skin = imagecreatefrompng("SKIN.png"); // Load a skin image
imagecopy($image, $skin, X-SPOT, Y-SPOT, CROP-LEFT, CROP-RIGHT, CROP-WIDTH, CROP-HEIGHT);

The skins I have are all in a sprite sheet, so cropping was really important for me. Chances are it wont be for you. You might want to scale your image though, so you can use imagecopyresized(...) to scale it too.

Step 3:
Drawing ttf text worked great for me! Simply upload a .ttf font to your website, and you can then draw text on the screen. Be careful with your positioning and sizing. You might need to fiddle with it.

imagettftext($image, FONT-SIZE-IN-PIXELS(usually), ROTATION, X-POSITION (from the left),
	Y-POSITION (from the bottom), imagecolorallocate($image, 255, 255, 255),
	"PATH-TO/FONT.ttf", "TEXT-TO-DISPLAY!");
// 255, 255, 255 is the color white.

Other font functions are available too! imagestring() doesn't require a font! But c'mon, make it pretty ;)

Step 4:
Saving uses the same function as in step 0!

imagepng($image, "PATH-TO/FILENAME.png"); //Overwrites by default

Then you can easily put PATH-TO/FILENAME.png into the image parameter of the tweet code, and you're good to go!