PHP DOM: change doctype of existing DOMDocument

Question

When creating a DOMDocument with DOMImplementation::createDocument(), you can specify a doctype as the third argument in the constructor. This doctype then gets "tied" to the document and you can retrieve it later with $document->doctype.

However, this is a read-only attribute (unlike encoding and version!).

Is there any way to change the doctype property of an existing DOMDocument object?

The only workaround I can think of is to create a new DOMDocument with the new doctype, and import then copy across the contents from the old one.

Answer 1

Turns out that in PHP (and therefore, I guess, in libxml) you can add and remove DOMDocumentType objects to an existing document just like regular nodes using methods like $document->insertBefore() and $document->removeChild(), etc.

Seems to work OK - changing it changes the $document->doctype value and everything. It's quite undocumented though, and contradicts the W3 DOM standard which says it shouldn't be possible (in DOM Level 2 at least). But handy to know that you can do it in PHP.

One downside is that saveHTML() does not "see" the new doctype, even though saveXML() does.

Answer 2

It also seems possible to call $document->loadXML() again even if the document is already populated. It wipes the document and regenerates it, but it is a way of changing the doctype variable on an existing object, and the new doctype does have an HTML representation unlike in my other solution.