I know very little of libxml2, but as far as I infer from http://xmlsoft.org/catalog.html and from the code in catalog.c, there is no way to specify a global catalog in a thread-local manner. This, by itself, makes it inviable to write a PHP function that allows replacing the default catalog unless some (probably expensive, since a catalog file must be read) on request startup/shutdown is used.

libxml2 supports per-document catalogs, but from what I see, your document must contain an oasis-xml-catalog processing instruction, like this:

<?xml version="1.0"?>
<?oasis-xml-catalog catalog="http://example.com/catalog.xml"?>
<!DOCTYPE doc PUBLIC "-//Example//DTD Document V1.0//EN"
                     "http://www.example.com/schema/doc.dtd">

There is another alternative, which is using an external entity loader (see http://xmlsoft.org/html/libxml-parser.html#xmlSetExternalEntityLoader ), but this is less convenient, though probably we could also expose some functions to deal with catalogs for use in the user-supplied callback.

I could implement this, but I'm hopping someone else with more libxml2 knowledge could tell if my analysis is correct.

> This, by itself, makes it inviable to write a PHP function that allows
> replacing the default catalog unless some (probably expensive, since a
> catalog file must be read) on request startup/shutdown is used.

The unless part doesn't actually make sense in the multi-threading versions of PHP.

Assign to self as I asked him to open this

> The user can change the default catalog behaviour by redirecting queries to its own set of catalogs. This can be done by setting the XML_CATALOG_FILES environment variable to a list of catalogs, an empty one should deactivate loading the default /etc/xml/catalog default catalog.

If I understand this right, you can use this code to set your own catalog:

setenv("XML_CATALOG_FILES=/path/to/catalog.xml");

I tried that, because I have to use xml catalog somehow, without overwriting the http stream wrapper, which is a non-threadsafe way to do this. However, the catalog.xml I specified using setenv seems not to be used by libxml2.

setenv("XML_CATALOG_FILES=/path/to/catalog.xml");

should normally work (depending which dialect that as) if it's set before PHP 
starts and loads the extension and the path to the catalog XML file does not 
contain any spaces (which has a special meaning).

See as well:

http://www.cafeconleche.org/books/effectivexml/chapters/47.html
http://www.chlab.ch/blog/archives/php/soap/cache-soap-envelope-schema-schema-
validation
http://wp.me/pLEEp-nlj

To change things at runtime, make use of libxml_set_external_entity_loader and 
implement a catalog parser your own. You then can use catalogs as you wish at 
runtime.