ป้องกัน spam ด้วย captcha คำถาม ทางคณิตศาสตร์ อย่างง่าย

<?php

session_start();

?>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">

<head>

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />

<title>php question verify</title>

<style type="text/css">

#verify_math{

display:block;

height:21px;

}

#verify_math span{

display:block;

height:21px;

width:30px;

position:relative;

float:left;

text-align:center;

line-height:20px;

color:#000;

}

#verify_math span.digital{

background:url(digi_img.jpg) left top no-repeat;

}

#i_verify{

position:relative;

height:15px;

width:35px;

text-align:center;

padding:0;

margin:0;

font-size:15px;

font-weight:bold;

font-family:Tahoma, Geneva, sans-serif;

}

</style>

</head>

 

<body>

<?php

$_SESSION['num_to_check'][0]=rand(1,9);

$_SESSION['num_to_check'][1]=rand(1,9);

?>

<form id="form1" name="form1" method="post" action="">

  <div id="verify_math">

  <span class="digital" style="background-position:<?=($_SESSION['num_to_check'][0]*-30)?>px 0px;"></span>

  <span>+</span>

  <span class="digital" style="background-position:<?=($_SESSION['num_to_check'][1]*-30)?>px 0px;"></span>

  <span>=</span>

  <span>

  <input name="i_verify" type="text" id="i_verify" maxlength="2" />

</span>

</div>

<br />

<br />

<input type="submit" name="button2" id="button2" value="Submit" />

</form>

 

 <?php

session_start();

if($_POST['button2']){

if(isset($_POST['i_verify']) && $_POST['i_verify']==@array_sum($_SESSION['num_to_check']) && $_POST['i_verify']>0 && trim($_POST['i_verify'])!=""){

echo "<span style="color:green;">Right</span>";

$_SESSION['num_to_check'][0]=rand(1,9);

$_SESSION['num_to_check'][1]=rand(1,9);

exit;

}else{

echo "<span style="color:red;">Wrong</span>";

$_SESSION['num_to_check'][0]=rand(1,9);

$_SESSION['num_to_check'][1]=rand(1,9);

exit;

}

}

?>

</body>

</html>