将用户输入的验证码值与会话中存储的期望值进行比较

$randomnr = rand(1000, 9999);
$_SESSION[\'randomnr2\'] = md5($randomnr);

if (strlen($captcha) !== ($_SESSION[\'randomnr2\'])) { 
    $error[\'captcha\'] = \"CAPTCHA error. Please try again\";
}

$captcha

if(md5($captcha) !== $_SESSION[\'randomnr2\']) {
  $error[\'captcha\'] = \"CAPTCHA error. Please try again\";
}

strlen()

echo strlen(42); 
echo strlen(\'42\');

if ($_SESSION[\'randomnr2\'] === (int)$captcha) {
   ... it matched ...
}

<div id=\'captcha_to_show\' style=\'border:1px solid silver;\'>gobldeygook</div>
<input name=\'captcha\' id=\'captcha\'>

$(\'document\').ready(function(){
  $(\'#captcha\').val($(\'#captcha_to_show\').html());
});