Add support for PCRE marks

It's a bit ridiculous that you can end up using a PHP
version from 2014 together with a decade old PCRE version.

That's an interesting way of looking at it. I don't know enough about
the specifics of this case, but the general idea feels close to a
conversation I had with a colleague the other day, in which he argued
that it doesn't make sense to restrict a new project to an already
out-of-date library just because that's what we're using elsewhere.
Since we were discussing CSS frameworks, any upgrade of old projects
would mean the site was getting a complete overhaul anyway - in the same
way that a PHP 5.6 package would be expected to come with either a
complete release of the distro, or some way of resolving "too new"
dependencies.

I also think that many PHP users don't realise just how tightly PHP's
extensions expose the specifics of a particular version of a 3rd-party
library. They might look at the documentation to check that a particular
function or option was added in PHP 5.4 or 5.5, but they probably
wouldn't even consider checking what version of PCRE (or libxml, etc,
etc) their system had, until they found something in their code
mysteriously "broke" on a particular environment. From that point of
view, the user-friendly approach for a PHP extension is to either
require a newer version of a library, or expose only the "lowest
common denominator" of functionality, so that the documentation can
confidently say "in PHP 5.6, you can do this".

Like I say, I don't know about the specifics of this case, so raising
the requirement may or may not be the right decision here, but it seems
like a useful line of thought.

Regards,

--
Rowan Collins
[IMSoP]

Ah, very good point. CentOS ships with 5.3, so a higher dependency for 3
minor versions greater makes sense.

• Trevor

Yea, I currently have "7.8-6.el6" on CentOS 6.5.
8.10 is too high a dependency

• Trevor

As Christopher pointed out on IRC, the distros shipping PCRE versions in
the 6.x-7.x range are also the ones that still use PHP 5.3 - by the time
they switch to 5.6 as default they'll also use a PCRE version that was
recent at that time.

I don't really have a problem with adding a few ifdefs for this, but I
think the low version requirement is bad for other reasons: There's a big
gulf in terms of both features and bug fixes between PCRE 6.6 and what PHP
currently bundles. It's a bit ridiculous that you can end up using a PHP
version from 2014 together with a decade old PCRE version.

Nikita