Clarify v4.x installation for php 7.x docs

[joseluisq]

Since I also faced the issue #60 with PHP 7.4.
This PR just clarify the v4.x installation docs for PHP 7.x on default branch's README according to https://pecl.php.net/package/memcache/4.0.5.2. Because the indicated v4.0.5.1 breaks my builds (but fixed by v4.0.5.2).

[nono303]

Hi @joseluisq,
related to #80 (comment), #76 (comment) it might be:

Please use

• latest version 4.x for < PHP 7.2 from branch NON_BLOCKING_IO_php7.
• latest version 8.x for PHP 7.2 7.3 & 7.4 from branch NON_BLOCKING_IO_php8.

[joseluisq]

I don't know because I got and error weeks ago when I used the latest 8.x with PHP 7.4 via pecl:

$ pecl install memcache
# pecl/memcache requires PHP (version >= 8.0.0), installed version is 7.4.13
# No valid packages found
# install failed

See Docker build https://travis-ci.com/github/joseluisq/alpine-php-fpm/jobs/455791248#L2338

However everything is fine when I lock my build with version 4.x:

$ pecl install memcache-4.0.5.2
# downloading memcache-4.0.5.2.tgz ...
# Starting to download memcache-4.0.5.2.tgz (75,061 bytes)
# .................done: 75,061 bytes
# 18 source files, building
# ....

See Docker build https://travis-ci.com/github/joseluisq/alpine-php-fpm/jobs/466062271#L2338

[nono303]

Ok!
to be precise, I was talking about compiling it (see https://github.com/nono303/PHP-memcache-dll#version-80-36d7181-branch-non_blocking_io_php8) and not for installing with pecl that check for some condition on dependencies versions

[joseluisq]

Ok!
to be precise, I was talking about compiling it (see https://github.com/nono303/PHP-memcache-dll#version-80-36d7181-branch-non_blocking_io_php8) and not for installing with pecl that check for some condition on dependencies versions

Probably I have misunderstood but I don't see any indication about neither compilation or pecl installation dependency.

This is an official repository for pecl-memcache plugin since 2019. 

This repository contains modified pecl-memcache plugin ported to PHP8,
which was originally developed for the need of hosting company in Slovakia (Websupport.sk). 

The latest release is 8.0 (released: 2020-12-06) with support for PHP 8.0.

Please use version 4.0.5.1 (released: 2020-12-19) for PHP 7.x from branch NON_BLOCKING_IO_php7.

See: https://github.com/websupport-sk/pecl-memcache/releases
See also release on PECL: https://pecl.php.net/package/memcache

Feel free to use it and post patches. 

Anyway that readme file needs to be more clear about it. That's why is confusing.
I will close this PR since the whole readme info needs to be updated.