<?php 
/**
* Captcha class
*/
class Captcha
{
  
  function __construct()
  {
    # code...
  }
}

<?php 
/**
* Captcha class
*/
class Captcha
{
  private $codeNum;  //验证码位数
  private $width;  //验证码图片宽度
  private $height;  //验证码图片高度
  private $img;  //图像资源句柄
  private $lineFlag;  //是否生成干扰线条
  private $piexFlag;  //是否生成干扰点
  private $fontSize;  //字体大小
  private $code;  //验证码字符
  private $string;  //生成验证码的字符集
  private $font;  //字体
  function __construct($codeNum = 4,$height = 50,$width = 150,$fontSize = 20,$lineFlag = true,$piexFlag = true)
  {
    $this->string = 'qwertyupmkjnhbgvfcdsxa123456789';  //去除一些相近的字符
    $this->codeNum = $codeNum;
    $this->height = $height;
    $this->width = $width;
    $this->lineFlag = $lineFlag;
    $this->piexFlag = $piexFlag;
    $this->font = dirname(__FILE__).'/fonts/consola.ttf';
    $this->fontSize = $fontSize;
  }
}

//创建图像资源  
public function createImage(){
    $this->img = imagecreate($this->width, $this->height);  //创建图像资源
    imagecolorallocate($this->img,mt_rand(0,100),mt_rand(0,100),mt_rand(0,100));  //填充图像背景（使用浅色）
  }

//创建验证码  
public function createCode(){
    $strlen = strlen($this->string)-1;
    for ($i=0; $i < $this->codeNum; $i++) { 
      $this->code .= $this->string[mt_rand(0,$strlen)];  //从字符集中随机取出四个字符拼接
    }
     $_SESSION['code'] = $this->code;  //加入 session 中
  
   //计算每个字符间距
    $diff = $this->width/$this->codeNum;
    for ($i=0; $i < $this->codeNum; $i++) { 
          //为每个字符生成颜色（使用深色）
     $txtColor = imagecolorallocate($this->img,mt_rand(100,255),mt_rand(100,255),mt_rand(100,255));
     //写入图像
      imagettftext($this->img, $this->fontSize, mt_rand(-30,30), $diff*$i+mt_rand(3,8), mt_rand(20,$this->height-10), $txtColor, $this->font, $this->code[$i]);
    }
  }

//创建验证码  
public function createCode(){
    $strlen = strlen($this->string)-1;
    for ($i=0; $i < $this->codeNum; $i++) { 
      $this->code .= $this->string[mt_rand(0,$strlen)];  //从字符集中随机取出四个字符拼接
    }
     $_SESSION['code'] = $this->code;  //加入 session 中
  
   //计算每个字符间距
    $diff = $this->width/$this->codeNum;
    for ($i=0; $i < $this->codeNum; $i++) { 
          //为每个字符生成颜色（使用深色）
     $txtColor = imagecolorallocate($this->img,mt_rand(100,255),mt_rand(100,255),mt_rand(100,255));
     //写入图像
      imagettftext($this->img, $this->fontSize, mt_rand(-30,30), $diff*$i+mt_rand(3,8), mt_rand(20,$this->height-10), $txtColor, $this->font, $this->code[$i]);
    }
  }

//创建干扰线条（默认四条）
public function createLines(){
    for ($i=0; $i < 4; $i++) { 
      $color = imagecolorallocate($this->img,mt_rand(0,155),mt_rand(0,155),mt_rand(0,155));  //使用浅色
      imageline($this->img,mt_rand(0,$this->width),mt_rand(0,$this->height),mt_rand(0,$this->width),mt_rand(0,$this->height),$color); 
    }
  }

//创建干扰点 （默认一百个点）
public function createPiex(){
    for ($i=0; $i < 100; $i++) { 
      $color = imagecolorallocate($this->img,mt_rand(0,255),mt_rand(0,255),mt_rand(0,255));
      imagesetpixel($this->img,mt_rand(0,$this->width),mt_rand(0,$this->height),$color);
    }
  }

 public function show()
  {
    $this->createImage();
    $this->createCode();
    if ($this->lineFlag) {  //是否创建干扰线条
      $this->createLines();
    }
    if ($this->piexFlag) {  //是否创建干扰点
      $this->createPiex();
    }
    header('Content-type:image/png');  //请求页面的内容是png格式的图像
    imagepng($this->img);  //以png格式输出图像
    imagedestroy($this->img);  //清除图像资源，释放内存
  }

public function getCode(){
    return $this->code;
  }
完整代码如下：
<?php 
/**
* Captcha class
*/
class Captcha
{
  private $codeNum;
  private $width;
  private $height;
  private $img;
  private $lineFlag;
  private $piexFlag;
  private $fontSize;
  private $code;
  private $string;
  private $font;
  function __construct($codeNum = 4,$height = 50,$width = 150,$fontSize = 20,$lineFlag = true,$piexFlag = true)
  {
    $this->string = 'qwertyupmkjnhbgvfcdsxa123456789';
    $this->codeNum = $codeNum;
    $this->height = $height;
    $this->width = $width;
    $this->lineFlag = $lineFlag;
    $this->piexFlag = $piexFlag;
    $this->font = dirname(__FILE__).'/fonts/consola.ttf';
    $this->fontSize = $fontSize;
  }

  public function createImage(){
    $this->img = imagecreate($this->width, $this->height);
    imagecolorallocate($this->img,mt_rand(0,100),mt_rand(0,100),mt_rand(0,100));
  }

  public function createCode(){
    $strlen = strlen($this->string)-1;
    for ($i=0; $i < $this->codeNum; $i++) { 
      $this->code .= $this->string[mt_rand(0,$strlen)];
    }
    $_SESSION['code'] = $this->code;
    $diff = $this->width/$this->codeNum;
    for ($i=0; $i < $this->codeNum; $i++) { 
      $txtColor = imagecolorallocate($this->img,mt_rand(100,255),mt_rand(100,255),mt_rand(100,255));
      imagettftext($this->img, $this->fontSize, mt_rand(-30,30), $diff*$i+mt_rand(3,8), mt_rand(20,$this->height-10), $txtColor, $this->font, $this->code[$i]);
    }
  }

  public function createLines(){
    for ($i=0; $i < 4; $i++) { 
      $color = imagecolorallocate($this->img,mt_rand(0,155),mt_rand(0,155),mt_rand(0,155));
      imageline($this->img,mt_rand(0,$this->width),mt_rand(0,$this->height),mt_rand(0,$this->width),mt_rand(0,$this->height),$color); 
    }
  }

  public function createPiexs(){
    for ($i=0; $i < 100; $i++) { 
      $color = imagecolorallocate($this->img,mt_rand(0,255),mt_rand(0,255),mt_rand(0,255));
      imagesetpixel($this->img,mt_rand(0,$this->width),mt_rand(0,$this->height),$color);
    }
  }

  public function show()
  {
    $this->createImage();
    $this->createCode();
    if ($this->lineFlag) {
      $this->createLines();
    }
    if ($this->piexFlag) {
      $this->createPiexs();
    }
    header('Content-type:image/png');
    imagepng($this->img);
    imagedestroy($this->img);
  }

  public function getCode(){
    return $this->code;
  }
}

session_start(); //开启session
$captcha = new Captcha();  //实例化验证码类(可自定义参数)
$captcha->show();  //调用输出

<div class="form-group">
 <div class="col-sm-12">
   <img src="admin/Captcha.php" alt="" id="codeimg" onclick="javascript:this.src = 'admin/Captcha.php?'+Math.random();">
   <span>Click to Switch</span>
 </div>
</div>