Detailed introduction to the principle of PHP verification code implementation

Expansion

We need to enable gd expansion. You can use the following code to check whether gd expansion is enabled.

<?phpecho "Hello World!!!!";echo phpinfo();?>

Then search the gd option on the browser Ctrl+F to verify whether you have installed this extension. If not, you need to install the extension yourself.

Background image

imagecreatetruecolor

Default generates black background

<?php
// 使用gd的
imagecreatetruecolor();创建一张背景图
$image = imagecreatetruecolor(100,30);// 在显示这张图片的时候一定要先声明头信息
header(&#39;content-type:image/png&#39;);

imagepng($image);// 释放资源，销毁执行对象imagedestroy($image);

imagecolorallocate

Create a fill color and use imagefill(image, x ,y,color) method to attach.

<?php
// 使用gd的
imagecreatetruecolor();创建一张背景图
$image = imagecreatetruecolor(100,30);// 生成填充色
$bgcolor = imagecolorallocate($image,255,255,255);// 将填充色填充到背景图上
imagefill($image,0,0,$bgcolor);// 在显示这张图片的时候一定要先声明头信息
header(&#39;content-type:image/png&#39;);

imagepng($image);// 释放资源，销毁执行对象imagedestroy($image);

imagepng

Before using this method, you must set the header information first, otherwise the image will not be displayed normally

imagedestory(image)

Timely release of resources will reduce the pressure on server requests.

Simple digital verification code

imagecolorallocate

Generate color information to facilitate processing later.

$fontcolor=imagecolorallocate($image,rand(0,255),rand(0,255),rand(0,255));

imagestring

Write the content information to the corresponding position of the image.

imagestring($image,$fontsize,$x,$y,$fontcontent,$fontcolor);

Increase recognition interference

Increase points

// 生成一些干扰的点，这里是200个
for($i=0;$i<200;$i++){    
$pointcolor = imagecolorallocate($image,rand(50,255),rand(50,255),rand(50,255));
    imagesetpixel($image,rand(0,100),rand(0,30),$pointcolor);
}

Increase lines

// 生成一些干扰线 这里是5个
for($i=0;$i<5;$i++){    // 设置为浅色的线，防止喧宾夺主
    $linecolor = imagecolorallocate($image,rand(50,255),rand(50,255),rand(50,255));
    imageline($image,rand(0,99),rand(0,29),rand(0,99),rand(0,29),$linecolor);

}

Mixed alphanumeric verification code

<?php
// 使用gd的imagecreatetruecolor();创建一张背景图
$image = imagecreatetruecolor(100,40);// 生成填充色
$bgcolor = imagecolorallocate($image,255,255,255);// 将填充色填充到背景图上
imagefill($image,0,0,$bgcolor);//////// 生成随机4位字母以及数字混合的验证码
for($i=0;$i<4;$i++){    
$fontsize = rand(6,8);    
$fontcolor = imagecolorallocate($image,rand(0,255),rand(0,255),rand(0,255));    
// 为了避免用户难于辨认，去掉了某些有歧义的字母和数字
    $rawstr = &#39;abcdefghjkmnopqrstuvwxyz23456789ABCDEFGHJKLMNOPQRSTUVWXYZ&#39;;    
    $fontcontent = substr($rawstr,rand(0,strlen($rawstr)),1);    
    // 避免生成的图片重叠
    $x += 20;    $y = rand(10,20);
    imagestring($image,$fontsize,$x,$y,$fontcontent,$fontcolor);   
}// 生成一些干扰的点，这里是200个for($i=0;$i<200;$i++){    
$pointcolor = imagecolorallocate($image,rand(50,255),rand(50,255),rand(50,255));
    imagesetpixel($image,rand(0,100),rand(0,30),$pointcolor);
}
// 生成一些干扰线 这里是4个for($i=0;$i<4;$i++){    
// 设置为浅色的线，防止喧宾夺主
    $linecolor = imagecolorallocate($image,rand(50,255),rand(50,255),rand(50,255));
    imageline($image,rand(0,99),rand(0,29),rand(0,99),rand(0,29),$linecolor);

}


header(&#39;content-type:image/png&#39;);

imagepng($image);// 释放资源，销毁执行对象imagedestroy($image);

Use verification code

The timing of opening the session

Note: The session must be opened at the beginning

The principle of verification

The verification process is The verification code entered by the client is compared with the verification code existing in the session field. That is:

if(isset($_REQUEST['checkcode'])){
        session_start();        
        if($_REQUEST['checkcode']==$_SESSION['checkcode']){            
        echo "<font color=&#39;green&#39;>Success!</font>"; 
        }else{            
        echo "<font color=&#39;red&#39;>Failed!</font>";    
        }        
        exit();
    }

Optimization verification

However, there is a disadvantage in simple verification, that is, the capitalization of letters is prone to errors. So we need to do a conversion to change all the values ​​​​entered by the user into lowercase.

if(strtolower($_REQUEST['checkcode'])==$_SESSION['checkcode']){···}

Small case

Generate verification code

Form verification
<?php
header("Content-Type：text/html;charset=utf8");        
if(isset($_REQUEST[&#39;checkcode&#39;])){
            session_start();            
            if(strtolower($_REQUEST[&#39;checkcode&#39;])==$_SESSION[&#39;checkcode&#39;]){                
            echo "<font color=&#39;green&#39;>Success!</font>"; 
            }else{                
            echo "<font color=&#39;red&#39;>Failed!</font>";    
            }            
            exit();
        }?>
        <!DOCTYPE html><html><head>
    <meta charset="utf-8" />
    <title>验证验证码信息</title>
    <script>
        function change(){
            document.getElementById("image_checkcode").src=&#39;./store.php?r=&#39;+Math.random();  
        }    </script></head><body><form action="./form.php" method="post"><p>验证码图片:</p>
        <img  src="/static/imghwm/default1.png"  data-src="./store.php?r=<?php echo rand();? alt="Detailed introduction to the principle of PHP verification code implementation" >"  class="lazy"  id="image_checkcode"    />
        <a href="javascript:void(0)" onclick="change()">看不清楚</a>
        <br/>请输入验证码<input type="text" name="checkcode" /><br /><p>
        <input type="submit" value="提交" /></p></form>
        </body></html>
Summary
Finally, let’s give a summary. 

Using php to create verification codes requires the support of gd extension. 
Use the imagecreatetruecolor method to generate the background color, and use imagefill to fill a color generated by imagecolorallocate. 
Use imagestring to combine the verification code and background image
Use imagesetpixel to add interference points
 
Use imageline to add interference lines

Before using session, you must enable the session_start() method at the beginning
Use JavaScript to dynamically modify The src of the verification code is used to meet the user's need to "change another one". 
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