I am dealing with a DTD file which has, for instance, the following parameter entity declaration:

The purpose of this declaration is so that the DTD can later include &ContentType; and this will be immediately replaced by the string, CDATA, which will then be immediately used in the declaration in which it appears. (Hence, "parameter" entity.) It's a common technique that's used to make DTDs more human-readable.

However, the DOMDocument::schemaValidate() method of PHP-7, which uses libxml.so under the hood, does not appear to understand this. It rejects the DTD with "invalid element type."

Looking at the source-code to this library on GitHub, I think that I have found (in parse.c) that it does not. But, I thought I'd ask ...

Obviously, I can circumvent the problem with a little text-editing, but it's a shame to lose that information and it seems strange to me that it doesn't work.

I am marking this thread as [SOLVED], not because I have yet found my answer ... (I still want one!) ... but because I found an effective work-around.

It turns out that "validateSchema" does not want a DTD, although it won't overtly refuse one. (Maybe it's supposed to allow both – I don't know.)

Anyhow – when I converted my DTD to an XSD file using an available tool, it was rejected as "not a schema file."

However, when I found and installed the "trang" tool to make a RelaxNG file, I actually got validation to work with the appropriate PHP method-call. Whew!