How to implement the principle of PHP verification code

PHP Verification Code Implementation Principle

With the continuous advancement of network technology, network security issues are becoming more and more important. Malicious users and bot attacks have become a number one problem for many websites. In order to prevent these attacks, many websites use verification code technology. This article will briefly introduce the principle of PHP verification code implementation.

What is a verification code

Verification code (CAPTCHA) is a test that distinguishes humans and machines. It is used to prevent automated programs from launching attacks on network systems or malicious registration. A verification code usually consists of a randomly generated number or combination of characters, and requires the user to manually enter the verification code to pass the verification. CAPTCHAs are widely used to prevent bots and malicious attacks because they are difficult for computers to understand.

PHP verification code implementation principle

In PHP, the most commonly used verification code implementation method is to turn a randomly generated string plus some interference into a picture, and then convert this The image is output to the front-end page, requiring the user to manually enter the verification code characters. The following will introduce the process of PHP verification code implementation in detail:

1. Generate a random string

First, we need to generate a random string as a verification code. You can use the PHP built-in functions rand() or mt_rand() to generate random integers and then convert those integers to ASCII characters. For example, the following code can generate a random string of length 6:

$characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
$randomString = '';
for ($i = 0; $i < 6; $i++) {
    $index = mt_rand(0, strlen($characters) - 1);
    $randomString .= $characters[$index];
}

where $characters is an optional character set, here numbers and uppercase and lowercase letters (62 characters in total) are selected; $randomString is Generated random string.

2. Create verification code image

Next, we need to convert the generated random string into an image and add interference factors. Here we can use the PHP extension library GD to achieve this. The GD extension provides a set of functions to create images, set colors, add text, etc. We can start by creating a blank image, set the width, height and background color, and then add random characters and noise lines to the image. For example, the following code can generate a 120x30 pixel verification code image:

header('Content-Type: image/png');
$image = imagecreate(120, 30);
$background_color = imagecolorallocate($image, 255, 255, 255);
imagefill($image, 0, 0, $background_color);

$line_color = imagecolorallocate($image, 64, 64, 64);
for ($i = 0; $i < 5; $i++) {
    imageline($image, 0, mt_rand(0, 30), 120, mt_rand(0, 30), $line_color);
}

$text_color = imagecolorallocate($image, 0, 0, 0);
imagestring($image, 5, 30, 8, $randomString, $text_color);

The first line specifies that the returned MIME type is image/png, because what we are going to output is a PNG format image; The imagecreate() function creates a 120x30 pixel blank image; the imagecolorallocate() function sets the background color, line color, and text color; the imageline() function draws 5 interference lines; the imagestring() function adds a random string.

3. Output verification code image

Finally, we need to output the generated verification code image to the front-end page so that users can see and enter the verification code. Here we can use PHP's imagepng() function to output the verification code image into a PNG format file. For example, the following code can output the verification code image to the front-end page:

imagepng($image);
imagedestroy($image);

The imagedestroy() function is used to release memory.

Summary

The implementation principle of PHP verification code is relatively simple. It mainly creates a blank image through the GD extension library, then adds random characters and interference factors, and finally outputs it as a picture. By introducing a CAPTCHA mechanism, we can improve website security by protecting it from bots and malicious attacks.

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