session_start();
getCode(4,60,20);
function getCode($num,$w,$h) {
  $code = "";
  for ($i = 0; $i < $num; $i++) {
    $code .= rand(0, 9);
  }
  //4位驗(yàn)證碼也可以用rand(1000,9999)直接生成
  //將生成的驗(yàn)證碼寫入session，備驗(yàn)證時(shí)用
  $_SESSION["helloweba_num"] = $code;
  //創(chuàng)建圖片，定義顏色值
  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, $gray);
  //畫邊框
  imagerectangle($im, 0, 0, $w-1, $h-1, $black);
  //隨機(jī)繪制兩條虛線，起干擾作用
  $style = array ($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, $y4, IMG_COLOR_STYLED);
  //在畫布上隨機(jī)生成大量黑點(diǎn)，起干擾作用;
  for ($i = 0; $i < 80; $i++) {
    imagesetpixel($im, rand(0, $w), rand(0, $h), $black);
  }
  //將數(shù)字隨機(jī)顯示在畫布上,字符的水平間距和位置都按一定波動(dòng)范圍隨機(jī)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);//輸出圖片
  imagedestroy($im);//釋放圖片所占內(nèi)存
}

<p>驗(yàn)證碼：<input type="text" class="input" id="code_num" name="code_num" maxlength="4" />
<img src="code_num.php" id="getcode_num" title="看不清，點(diǎn)擊換一張" align="absmiddle"></p>
<p><input type="button" class="btn" id="chk_num" value="提交" /></p>

$(function(){
  //數(shù)字驗(yàn)證
  $("#getcode_num").click(function(){
    $(this).attr("src",'code_num.php?' + Math.random());
  });
  ...
});

$(function(){
  ...
  $("#chk_num").click(function(){
    var code_num = $("#code_num").val();
    $.post("chk_code.php?act=num",{code:code_num},function(msg){
      if(msg==1){
        alert("驗(yàn)證碼正確！");
      }else{
        alert("驗(yàn)證碼錯(cuò)誤！");
      }
    });
  });
});

session_start();
$code = trim($_POST['code']);
if($code==$_SESSION["helloweba_num"]){
  echo '1';
}