I've provided my full set of updated files, zipped, and also the diff patches for the two files that were altered: Simple-LDAP-Login.php and Simple-LDAP-Login-Admin.php

I hope this helps someone else trying to get LDAP going with WordPress.

Download a zipped version of the updated plugin here:
simpleLDAPupdated_for_WP3.5.zip

Patches for the two files that were altered are given below.
Patch for Simple-LDAP-Login.php:

@@ -8,7 +8,7 @@ Author: Clifton H. Griffin II

 Author URI: http://clifgriffin.com

 */

 require_once( WP_PLUGIN_DIR."/simple-ldap-login/adLDAP.php");

-//require_once( ABSPATH . WPINC . '/registration.php'); //removed for WP5.3.5.1. compatibility

+require_once( ABSPATH . WPINC . '/registration.php');

 

 //Admin

 function simpleldap_menu()

@@ -18,10 +18,7 @@ function simpleldap_menu()

 

 function simpleldap_admin_actions()

 {

-   add_options_page("Simple LDAP Login", "Simple LDAP Login", "create_users", "simple-ldap-login", "simpleldap_menu");

-  // per http://tumbledesign.com/fix-notice-has_cap-was-called-with-an-argument-that-is-deprecated-since-version-2-0-in-wordpress/

-  //changed this for WP5.3.5.1. compatibility

-   // add_options_page("Simple LDAP Login", "Simple LDAP Login", 10, "simple-ldap-login", "simpleldap_menu");

+    add_options_page("Simple LDAP Login", "Simple LDAP Login", 10, "simple-ldap-login", "simpleldap_menu");

 }

 function simpleldap_activation_hook()

 {

@@ -56,7 +53,6 @@ $sll_options=array(

 	"domain_controllers"=>explode(";",get_option("simpleldap_domain_controllers")),	

 );

 

-//"$sll_options=".print_r($sll_options); //debug

 //For OpenLDAP

 $ar_ldaphosts = explode(";",get_option("simpleldap_domain_controllers"));

 $ldaphosts = ""; //string to hold each host separated by space

@@ -68,7 +64,6 @@ foreach ($ar_ldaphosts as $host)

 }

 define ('LDAP_HOST', $ldaphosts);

 define ('LDAP_PORT', 389);

-//define ('LDAP_PORT', 636); //I prefer 636 as default

 define ('LDAP_VERSION', 3);

 define ('BASE_DN', get_option('simpleldap_base_dn'));

 define ('LOGIN', get_option("simpleldap_ol_login"));

@@ -104,9 +99,7 @@ function sll_authenticate($user, $userna

 	$auth_result = sll_can_authenticate($username, $password);

 	if($auth_result == true && !is_a($auth_result, 'WP_Error'))

 	{

-			//$user = get_userdatabylogin($username); //http://codex.wordpress.org/Function_Reference/get_userdatabylogin

-			$user = get_user_by('login', $username);//changed because get_userdatabylogin is deprecated

-			

+			$user = get_userdatabylogin($username);

 	

 			if ( !$user || (strtolower($user->user_login) != strtolower($username)) ) 

Patch for Simple-LDAP-Login-Admin.php

  
 //If admin options updated (uses hidden field)
-if ((isset($_POST['stage']))&& ($_POST['stage'] == 'process') )
-//if ($_POST['stage'] == 'process') 
+if ($_POST['stage'] == 'process') 
 {
     update_option('simpleldap_account_suffix', $_POST['account_suffix']);
 	update_option('simpleldap_base_dn', $_POST['base_dn']);
@@ -104,21 +103,20 @@ if ((isset($_POST['stage']))&& ($_POST['
 	
 }
 //Test credentials
-elseif ((isset($_POST['stage']))&& ($_POST['stage'] == 'test') )
-//elseif ($_POST['stage'] == 'test') 
+elseif ($_POST['stage'] == 'test') 
 {
 	global $bool_test;
 	
 	//Temporarily change security mode for test. Store old setting.
 	$temp_holder = get_option("simpleldap_security_mode");
-	update_option("simpleldap_security_mode", "security_low");
-	//	update_option("simpleldap_security_mode", "security_high");
+	update_option("simpleldap_security_mode", "security_high");
 	
 	$test_user = wp_authenticate($_POST['test_username'],$_POST['test_password']);
 	
 	//Restore security mode setting.
 	update_option("simpleldap_security_mode", $temp_holder);
-	if ((isset($test_user->ID) )&&($test_user->ID > 0))
+	
+	if ($test_user->ID > 0)
 	{
 		$bool_test = 1;
 	}