session_start(); 
getCode(4,60,20); 
  
function getCode($num,$w,$h) { 
  $code = ""; 
  for ($i = 0; $i < $num; $i++) { 
    $code .= rand(0, 9); 
  } 
  //4位验证码也可以用rand(1000,9999)直接生成 
  //将生成的验证码写入session，备验证时用 
  $_SESSION["helloweba_num"] = $code; 
  //创建图片，定义颜色值 
  header("Content-type: image/PNG"); 
  $im = imagecreate($w, $h); 
  $black = imagecolorallocate($im, 0, 0, 0); 
  $gray = imagecolorallocate($im, 200, 200, 200); 
  $bgcolor = imagecolorallocate($im, 255, 255, 255); 
  //填充背景 
  imagefill($im, 0, 0, $gray); 
  
  //画边框 
  imagerectangle($im, 0, 0, $w-1, $h-1, $black); 
  
  //随机绘制两条虚线，起干扰作用 
  $style = array ($black,$black,$black,$black,$black, 
    $gray,$gray,$gray,$gray,$gray 
  ); 
  imagesetstyle($im, $style); 
  $y1 = rand(0, $h); 
  $y2 = rand(0, $h); 
  $y3 = rand(0, $h); 
  $y4 = rand(0, $h); 
  imageline($im, 0, $y1, $w, $y3, IMG_COLOR_STYLED); 
  imageline($im, 0, $y2, $w, $y4, IMG_COLOR_STYLED); 
  
  //在画布上随机生成大量黑点，起干扰作用; 
  for ($i = 0; $i < 80; $i++) { 
    imagesetpixel($im, rand(0, $w), rand(0, $h), $black); 
  } 
  //将数字随机显示在画布上,字符的水平间距和位置都按一定波动范围随机生成 
  $strx = rand(3, 8); 
  for ($i = 0; $i < $num; $i++) { 
    $strpos = rand(1, 6); 
    imagestring($im, 5, $strx, $strpos, substr($code, $i, 1), $black); 
    $strx += rand(8, 12); 
  } 
  imagepng($im);//输出图片 
  imagedestroy($im);//释放图片所占内存 
}