通过PHP进行LDAP身份验证仍然失败

我正在尝试使用PHP和LDAP对Active Directory进行身份验证。不幸的是，当我试图通过表单发送用户名时，它失败了。即使我在认证函数中指定了$userName和$passWord，该函数仍然返回false。我做错了什么?

<?php
session_start();
function authenticate($userName,$passWord) {
//Active Directory Server
$ldap_host = "example.com";
//Active Directory DN
$ldap_dn = "DC=myCompany,DC=org";
//Connect to AD Server
$ldap = ldap_connect($ldap_host)
    or die("Could not connect to LDAP server.");
utf8_decode($passWord);
//Specify Active Directory  
ldap_set_option ($ldap, LDAP_OPT_REFERRALS, 0);
ldap_set_option($ldap, LDAP_OPT_PROTOCOL_VERSION, 3);
//Verify userName and passWord
if($bind = @ldap_bind($ldap, $userName, $passWord)) {
    //if valid
    $ldapResults = 'Bind Successful';
    return true;        
} else {
    //invalid username or password
    return false;
}
};
if(isset($_POST['userName'])) {
//authenticate via function
if(authenticate($_POST['userName'],$_POST['passWord']))
{
    //successfully authenticated (scroll down)
    header("Location: example.com");
    $error = null;
} else {
    //failed authentication
    $error = 1;
}
}
?>

<form method="post" id="loginForm" action="login.php">
            <ul>
                 <li>
                    <input type="text" id="userName" name="userName" />
                </li>
                <li>
                    <input type="password" id="passWord" name="passWord" />
                </li>
                <li>
                    <input type="submit" name="submit" value="Log In"></input>
                </li>
            </ul>
 </form>