Description:
------------
In my Arch Linux I activated PDO MySql in the php.ini to connect to Mariadb:
extension=pdo_mysql

And I changed the socket path:
pdo_mysql.default_socket=/run/mysqld/mysqld.sock


When I connect to the database giving an empty hostname, PHP tries to connect to /tmp/mysql.sock ignoring the php.ini setting. (I saw this using strace)

Test script:
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<?php
// the following two lines work
new \PDO("mysql:", "http", "");
new \PDO("mysql:host=localhost", "http", "");

// this line fails
new \PDO("mysql:host=", "http", "");

Expected result:
----------------
Three successful connections, no output.

Actual result:
--------------
PHP Fatal error:  Uncaught PDOException: SQLSTATE[HY000] [2002] No such file or directory in /srv/http/databases.php:7
Stack trace:
#0 /srv/http/databases.php(7): PDO->__construct('mysql:host=', 'http', '')
#1 {main}
  thrown in /srv/http/databases.php on line 7