Is it possible to open Windows Explorer using PHP?
No! PHP is a language that runs on server-side; In a general context you will not be able to interact with native resources on the client-side, that is, in the user's browser;
Is it possible to open Windows Explorer using Javascript?
Using Internet Explorer (this is probably not possible with other browsers), you can use some ActiveX features, but I only tried doing this with HTML saved on the machine itself; Using something like:
<script type="text/javascript">
window.open("nomeDoArquivo.html", "NomeDaNovaJanela");
</script>
I do not know the context to which you need to use this solution, but from the security point of view this is not advisable; In addition to the fact that most browsers do not allow actions like this, you will face several restrictions with anti-virus software that consider such actions as a threat;
I recommend thinking of another approach to solving your problem; Handling directories on the client does not seem to be an approach to Web-based solutions, for the simple fact that you can not guarantee that a particular directory exists on the client;