Creating a CAPTCHA class in PHP

Creating a CAPTCHA class in PHP can help you add a level of security to your web forms by ensuring that the submitted data comes from a human, rather than an automated system. 

<?php

class SimpleCaptcha {
    private $code;
    private $width;
    private $height;
    
    public function __construct($width = 150, $height = 40) {
        $this->width = $width;
        $this->height = $height;
        $this->code = $this->generateCode();
    }
    
    // Generate a random code for CAPTCHA
    private function generateCode($length = 6) {
        $characters = 'ABCDEFGHJKLMNPQRSTUVWXYZ23456789';  // Avoid ambiguous characters
        $code = '';
        for ($i = 0; $i < $length; $i++) {
            $code .= $characters[rand(0, strlen($characters) - 1)];
        }
        return $code;
    }
    
    // Render CAPTCHA image
    public function render() {
        // Create an image
        $image = imagecreatetruecolor($this->width, $this->height);
        
        // Allocate colors
        $bgColor = imagecolorallocate($image, 255, 255, 255); // white
        $textColor = imagecolorallocate($image, 0, 0, 0);     // black
        $lineColor = imagecolorallocate($image, 64, 64, 64);  // grey
        $pixelColor = imagecolorallocate($image, 0, 0, 255);  // blue
        
        // Fill the background
        imagefilledrectangle($image, 0, 0, $this->width, $this->height, $bgColor);
        
        // Add some random lines for obfuscation
        for ($i = 0; $i < 5; $i++) {
            imageline($image, 0, rand() % $this->height, $this->width, rand() % $this->height, $lineColor);
        }
        
        // Scatter some random dots
        for ($i = 0; $i < 500; $i++) {
            imagesetpixel($image, rand() % $this->width, rand() % $this->height, $pixelColor);
        }
        
        // Add the text
        $fontFile = __DIR__ . '/arial.ttf'; // Path to a TTF font file
        $fontSize = 20;
        $textBox = imagettfbbox($fontSize, 0, $fontFile, $this->code);
        $textWidth = $textBox[2] - $textBox[0];
        $textHeight = $textBox[7] - $textBox[1];

        $x = ($this->width - $textWidth) / 2; // Center the text horizontally
        $y = ($this->height - $textHeight) / 2; // Center vertically, adjust as needed

        imagettftext($image, $fontSize, 0, $x, $y, $textColor, $fontFile, $this->code);
        
        // Output the image
        header("Content-type: image/png");
        imagepng($image);
        imagedestroy($image);
    }
    
    // Get the current CAPTCHA code
    public function getCode() {
        return $this->code;
    }
}

// Usage example
$captcha = new SimpleCaptcha();
session_start();
$_SESSION['captcha'] = $captcha->getCode();
$captcha->render();

Explanation:

1. Class Initialization:
   • The class SimpleCaptcha is initialized with a default width and height for the CAPTCHA image.
   • A random code is generated when an object of the class is instantiated.
2. Code Generation:
   • The generateCode() method creates a random sequence of characters excluding ambiguous ones like ‘O’ and ‘0’.
3. Image Rendering:
   • imagecreatetruecolor() is used to create a blank image with the specified dimensions.
   • Colors are allocated for the background, text, random lines, and pixels.
   • The background is filled with a light color (white in this case).
   • Random lines and pixels are added to make the CAPTCHA more difficult for bots to read.
4. Text Rendering:
   • The code is drawn onto the image using imagettftext() for a better font rendering.
   • Text is centered on the image for aesthetic appeal.
5. Output:
   • The image is output as a PNG file directly to the browser.
6. Session Storage:
   • The generated code is saved in a session to verify against user input later.

Considerations:

• Ensure that the TTF font file (arial.ttf in this case) is available and correct.
• Depending on your environment, PHP GD library should be installed and configured to handle image creation and manipulation.
• You should adjust the fonts, colors, and complexity to balance readability for humans and difficulty for automated systems.