Please provide LICENSE #273
Having the licence file distributed in PEAR places it in a completely different directory away from the actual source code, so it's basically useless. That's why the package.xml and every file header points to the licence file on Github.
Changing the name would require me to change every file header, and instantly break links in the header of all existing distributions, so I'm not going to do that. Also, there is absolutely no requirement to name the licence file LICENCE. The licence file can be named anything you want (especially given there are 2 spellings). All naming and availability is provided for convenience.
Just to clarify: I am not currently in breach of my own licence just because the file is called licence.txt and it is not in the package.xml file.
Having said all that, I'll add the licence.txt entry into the package.xml file considering you've asked for it. I just want to be clear that this change does not affect the licensing of PHP_CodeSniffer in any way.