The Bulgarian translation of the manual is not being published.  The English language page is available at http://php.net/manual/en/function.exec.php and several other languages are available, but not Bulgarian.

The same happens to pages in the /pl/ section, so it would seem there's something wrong with localized manual pages in general.

It would probably be beneficial if localized manual pages were getting redirected to /en/ in case of a missing page.

Just to provide some more details for context.  The builds of the Bulgarian translation of the manual have not been done since 2010 [1].  Similarly, several other unmaintained translations are not built [2].  This is because the sources for those translations are not being kept up to date [3].

Due to the way the PHP manual is distributed to the php.net website mirror servers, the old versions of those translations were mistakenly still being published; mostly with the English language version of the pages where any translation did not occur.  This resulted in the "translated" versions of the manual containing mostly English with some vastly out of date translated pages.  

Recently, the pushing of those inactive, out-of-date translations to the website mirrors was stopped.  This results in the 404 Not Found reported in this bug report.  The expectation is that Google will stop serving those pages in search results eventually; hopefully sooner rather than later.

sinus+php at sinpi dot net's suggestion is great, and should be done as a good way of directing visitors hitting those pages to their English counterparts.

[1] https://github.com/php/web-php/commit/4dfcbff072
[2] https://github.com/php/web-php/blame/master/include/languages.inc#L50-L85
[3] http://doc.php.net/revcheck.php

I have updated the error page in this case to be more clear that the page (from an inactive manual translation) has gone away deliberately and added links to alternative pages (i.e. other languages).

Thanks for your bug report and suggestions.