How can PHP implement captcha functionality?

To implement captcha functionality, you can use the GD library in PHP or utilize a third-party captcha library.

The steps to implement the captcha feature using the GD library are as follows:

1. Generate a verification code image by creating a true color image of a specified size using the imagecreatetruecolor() function, and choose background and font colors.
2. Create a random string by shuffling a specified character set using the str_shuffle() function, then truncate it to a specified length as a verification code.
3. Write the verification code string onto the image. Use the imagestring() function to write the generated verification code string onto the image.
4. Add interference elements. Use functions like imagesetpixel() and imageline() to randomly add pixels and lines on the image as interference elements.
5. Output the image. Use the header() function to set the response header, informing the browser that the output is an image, and use functions like imagepng() to output the image.
6. Validate user input by storing the generated verification code in the session and comparing it with the user input to verify its correctness.

The steps to implement captcha functionality using a third-party captcha library are as follows:

1. Install a third-party captcha library. You can use tools like Composer to install some commonly used captcha libraries.
2. Set up the verification code library. Follow the documentation of the third-party verification code library to configure settings such as colors, length, font, etc.
3. Generate and output a verification code. Call the appropriate function to generate the verification code and output it to the HTML page.
4. Verify user input. Use the documentation from the third-party verification library to call the appropriate function to confirm if the user inputted code is correct.

The above are two common methods used to implement the verification code feature, the specific choice depends on individual needs and the functionality of the third-party library being used.