For Windows Platform/LDAPS/Active Directory, aside from enabling php_ldap.dll in PHP.ini, you will also have to add trusted RootCA file reference. That's because OpenLDAP, by default, is looking for "C:\openldap\sysconf\ldap.conf", which doesn't exists. Create the missing patch/file and inside the file: TLS_CACERT <explicit file path to RootCA of Active Directory issuer>.
If you do not like default C:\ path above, create an environment variable, LDAPCONF=<file path>. Restart IIS for change to take effect.
[Example] - assume PHP is under D:\PHP
1. create new, empty file D:\PHP\ldap.conf
2. add inside file: TLS_CACERT D:\PHP\rootCA.pem
3. copy your Active Directory's Root CA cert file (rootCA.pem) to D:\PHP
4. add Environment Variable: LDAPCONF=D:\PHP\ldap.conf
5. Restart IIS: iisreset /restart