This is just a guess, but I believe that MediaWiki uses the term "namespace" in
a different way, when compared to PHP.
https://www.mediawiki.org/wiki/Namespaces
http://php.net/manual/en/language.namespaces.definition.php
PHP introduced Namespaces at PHP 5.3+
Prior to that, the word "namespace" had no meaning in PHP. It may be worth
checking to see if your hosting provider upgraded PHP recently.
No trouble using PHP 5.4 and MW 1.21, or PHP 5.5 and MW 1.26 (these are my only two
installations)
I'm a big fan of keeping the software as up-to-date as possible. In the PHP world
this means PHP 7+ but it also means breaking changes for some of the previously deprecated
"features" from the dustbin of PHP. Docs here:
http://php.net/manual/en/migration70.incompatible.php
PHP 5 is still supported at PHP 5.5+ or PHP 5.6+
HTH, Ray