How to generate image verification code in PHP

session_start ();
/*定义头文件为图片*/
header("Content-type: image/png");
/*生成验证码*/
/*创建图片设置字体颜色*/
$im = imagecreate($w, $h);
$red = imagecolorallocate($im, 255, 255, 255);
$white = imagecolorallocate($im, 255, 255, 255);
/*随机生成两个数字*/
$num1 = rand(1, 20);
$num2 = rand(1, 20);
$_SESSION ["administratorConfirmCode"] = $num1+$num2;
/*设置图片背景颜色*/
$gray = imagecolorallocate($im, 118, 151, 199);
$black = imagecolorallocate($im, mt_rand(0, 100), mt_rand(0, 100), mt_rand(0, 100));
/*创建图片背景*/
imagefilledrectangle($im, 0, 0, 100, 24, $black);
/*在画布上随机生成大量点*/
for ($i = 0; $i < 80; $i++) {
  imagesetpixel($im, rand(0, $w), rand(0, $h), $gray);
}
/*将计算验证码写入到图片中*/
imagestring($im, 5, 5, 4, $num1, $red);
imagestring($im, 5, 30, 3, "+", $red);
imagestring($im, 5, 45, 4, $num2, $red);
imagestring($im, 5, 70, 3, "=", $red);
imagestring($im, 5, 80, 2, "?", $white);
/*输出图片*/
imagepng($im);
imagedestroy($im);

session_start ();
/*设置文件头为图片输出*/
Header("Content-type: image/JPEG");
/*调用生成验证码函数*/
$str="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz01234565789";
$result="";
for($i=0;$i<$length;$i++){
  $num[$i]=rand(0,61);
  $result.=$str[$num[$i]];
}
$text = $result;
$_SESSION ["administratorConfirmCode"] = $text;
/*设置图片的宽度和高度*/
$im_x = $w;
$im_y = $y;
/*创建图片*/
$im = imagecreatetruecolor($im_x,$im_y);
$text_c = ImageColorAllocate($im, mt_rand(0,100),mt_rand(0,100),mt_rand(0,100));
$tmpC0=mt_rand(100,255);
$tmpC1=mt_rand(100,255);
$tmpC2=mt_rand(100,255);
$buttum_c = ImageColorAllocate($im,$tmpC0,$tmpC1,$tmpC2);
imagefill($im, 16, 13, $buttum_c);
/*字体文件*/
$font = _WEB_DIR_.&#39;/font/comic.ttf&#39;;
for ($i=0;$i<strlen($text);$i++){
  $tmp =substr($text,$i,1);
  $array = array(-1,1);
  $p = array_rand($array);
  $an = $array[$p]*mt_rand(1,10);//角度
  $size = 28;
  imagettftext($im, $size, $an, 15+$i*$size, 35, $text_c, $font, $tmp);
}
/*将字符写入文件中*/
$distortion_im = imagecreatetruecolor ($im_x, $im_y);
imagefill($distortion_im, 16, 13, $buttum_c);
for ( $i=0; $i<$im_x; $i++) {
  for ( $j=0; $j<$im_y; $j++) {
    $rgb = imagecolorat($im, $i , $j);
    if( (int)($i+20+sin($j/$im_y*2*M_PI)*10) <= imagesx($distortion_im)&& (int)($i+20+sin($j/$im_y*2*M_PI)*10) >=0 ) {
      imagesetpixel ($distortion_im, (int)($i+10+sin($j/$im_y*2*M_PI-M_PI*0.1)*4) , $j , $rgb);
    }
  }
}
/*干扰元素点的数量*/
$count = 160;
/*创建干扰元素点*/
for($i=0; $i<$count; $i++){
  $randcolor = ImageColorallocate($distortion_im,mt_rand(0,255),mt_rand(0,255),mt_rand(0,255));
  imagesetpixel($distortion_im, mt_rand()%$im_x , mt_rand()%$im_y , $randcolor);
}
/*创建干扰线条*/
$rand = mt_rand(5,30);
$rand1 = mt_rand(15,25);
$rand2 = mt_rand(5,10);
for ($yy=$rand; $yy<=+$rand+2; $yy++){
  for ($px=-80;$px<=80;$px=$px+0.1){
    $x=$px/$rand1;
    if ($x!=0){
      $y=sin($x);
    }
    $py=$y*$rand2;
    imagesetpixel($distortion_im, $px+80, $py+$yy, $text_c);
  }
}
/*以PNG格式将图像输出到浏览器*/
ImagePNG($distortion_im);
/*销毁图像*/
ImageDestroy($distortion_im);
ImageDestroy($im);