Steps to reproduce the issue:

1. If Cache or Session Handlers are File and Database - connection is fine.
2. Setting up php-memcache (also php-memcached) to connect via tcp (local, 11211) - connection is fine.
3. It is impossible to even save the settings for connection via unix socket at Global configuration->System dash board (error: "invalid port" when trying to set port to 0 (zero)".
4. If settings are adjusted manually directly in configuration.php - loss of connection and errors follow.

Expected result - to get connection with the following settings:

Memcache(d) Server Host = unix:///var/run/memcached/memcached.sock
The following path to socket: /var/run/memcached/memcached.sock setting up memcached (pecl)
Memcache(d) Server Port = 0 (zero)

Permissions to /var/run/memcached 0775

Memcached.conf:
-s /path/to/memcache.socket
-a 0777

Actual result - connection lost with the errors as follows:

1. Warning: session_start(): Failed to initialize storage module: memcache (path: unix:///var/run/memcached/memcached.sock:0) in /path/www/test/libraries/joomla/session/handler/native.php on line 260
   Error: Failed to start application: Failed to start the session

or 2) Warning: session_start(): Failed to parse session.save_path (error at offset 0, url was '/var/run/memcached/memcached.sock:0') in /home/path/www/test/libraries/joomla/session/handler/native.php on line 260

Warning: session_start(): Failed to initialize storage module: memcache (path: /var/run/memcached/memcached.sock:0) in /home/path/www/test/libraries/joomla/session/handler/native.php on line 260
Error: Failed to start application: Failed to start the session

System information:

System: Ubuntu 18.04, Apache 2.4, php 7.3.1, Memcached server 1.5.12, php-pecl-memcache 3.0.9-dev, php-pecl-memcached 3.1.3).

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