How to correctly use `.user.ini`-file in PHP?

In my working directory I have two files: index.php and .user.ini:

.user.ini:

display_errors=on

; http://be2.php.net/manual/en/filter.configuration.php#ini.filter.default
;filter.default = "full_special_chars"

index.php:

<?php
//ini_set("display_errors", "on");

// Use `.user.ini` file:
// http://php.net/manual/en/configuration.file.per-user.php
echo "Setting for 'user_ini.filename': " . ini_get("user_ini.filename");
echo "\n\n";


// It takes up to five minutes, until `.user.ini` is re-read:
echo "Setting for 'user_ini.cache_ttl': " . ini_get("user_ini.cache_ttl");
echo "\n\n";

// http://php.net/manual/en/function.ini-get.php
echo "Setting for 'display_errors': " . ini_get("display_errors");
echo "\n\n";
echo "Setting for 'filter.default': " . ini_get("filter.default");
echo "\n\n";

// php -S localhost:8000
// http://localhost:8000/

Using the above .user.ini-file (in my working directory) I expect the "Setting for 'display_errors': " having a value of on or 1, but it's empty.

How to correctly change settings using the .user.ini-file?

running php --ini results in

Configuration File (php.ini) Path: /etc/php5/cli
Loaded Configuration File:         /etc/php5/cli/php.ini
Scan for additional .ini files in: /etc/php5/cli/conf.d
Additional .ini files parsed:      /etc/php5/cli/conf.d/05-opcache.ini,
/etc/php5/cli/conf.d/10-pdo.ini,
/etc/php5/cli/conf.d/20-json.ini,
/etc/php5/cli/conf.d/20-readline.ini

which does not contain my .user.ini-file.

Explicitly adding the .user.ini-file works:

php --php-ini .user.ini index.php

but I'd like it to be automatically read when running the script from a given folder.

In the documentation it says:

These files are processed only by the CGI/FastCGI SAPI

and

If you are using Apache, use .htaccess files for the same effect.

So if you run PHP as an Apache module, from the commandline, or using the builtin-Server, the .user.ini-files are not processed.

To find out your Server API, simply generate a PHP script with the following content and call it in your webbrowser:

<? phpinfo();