PHP - 与我的验证码系统中的==相比，相同的文本字符串返回false

Ok, I've been struggling for days with this issue with my captcha system. First it always was returning false but if you refreshed the page it would log you in. Then I pinpointed where the error was occurring and simplified the code (removed the unneeded database queries and $_Sessions), and now I can't even sign in if I refresh the page after it returning false like before and the error appeared to be occurring here (before and currently):

if(strtolower($characters)==strtolower($_POST["security_code"])){

Even though the text from $characters and $_POST["security_code"] are the exact same, I always get it to return false. This is an example test.php I made to show it not working. I've tried doing stuff with sessions to fix this but I might of just not done it correctly and that's what needs to be done.

<?php
$possible = '23456789bcdfghjkmnpqrstvwxyz';
$characters = ''; 
$i = 0;

while($i < 8){
    $characters .= substr($possible, mt_rand(0, strlen($possible)-1), 1);
    $i++;
}

if(strtolower($characters)==strtolower($_POST["security_code"])){
    $captcha_solved = 'true';
}else{
    $captcha_solved = 'false';
}

?>
<form name="example" action="test.php" method="post">
     <input name="security_code" size="10" type="text">
     <input name="Login" value="Login" type="submit" value="submit">
</form>

<? echo($characters."-".$captcha_solved); ?>