redirect missing php page to aspx page - CustomErrors won't work?

[Les Caudle]

I can set up CustomErrors to redirect a missing aspx page to Register.aspx like:

<customErrors mode="RemoteOnly" defaultRedirect="ErrorPage.aspx">
<error statusCode="404" redirect="Register.aspx"/>
</customErrors>

I've got a client who needs a missing php page to redirect to Register.aspx.

This gets caught by IIS, not ASP, so instead of redirecting, I get a standard
404 error msg.

What is the simplest way to redirect this?