If you’re struggling to get the LDAP extension to work in PHP and IIS6 then you may want to read on. In my situation both the web server and PHP were confirmed to be working fine and reading the correct php.ini. But even with the correct line enabled in the php.ini file, LDAP would refuse to show in phpinfo() output.
After some searching of my own, it turns out that IIS6 on Windows Server 2003 (possibly XP too) will not properly read the PATH variable. So if you add the path to your PHP directory to the end of PATH, the DLLs required are still not found. This looks to affect IIS6 specifically as filesystem calls to the same DLLs did find them.
The solution – add the path to your PHP install to the start or earlier in the PATH variable and restart IIS. In my experience you will now see the LDAP options appear in your phpinfo() output.