帮忙看下这代码哪里有问题，为什么大小写转换不了呢？
<?php
if(isset($_REQUEST['authcode'])){
session_start();
if (strtolower($_REQUEST['authcode'])==$_SESSION['authcode'])
{
header('Content-type: text/html; charset=UTF8');
echo '<font color="#0000CC">输入正确</font>';
}
else{
echo '<font color="#CC0000"><b>输入错误</b></font>';
echo $_REQUEST['authcode'];
}
exit;
}
?>
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>确认验证码</title>
</head>
<body>
<form method="post" action="./form.php">
<p>验证码图片：<img border="1" src="./captcha.php"?r=<?php echo rand();?>"width=100,height=30";</p>
<p>请输入图片中的内容：<input type="text" name="authcode" value=""></p>
<p><input type="submit" value="提交" style="padding:6px 20px;"></p>
</form>
</body>
</html>
