Is it possible to use environment variables in php.ini?

According to the PHP docs, environment-variables can indeed be used in the configuration file.

It doesn’t say anything about what syntax to use, but it is the same as with Apache’s configuration file which uses *nix syntax. So for example, if you want PHP to use the system temp-directory, you would use this:

upload_tmp_dir = ${Temp}

You can confirm that it is active with the following script:

<?php
    echo "ini:  " . ini_get('upload_tmp_dir') . "\n";
    echo "env:  " . sys_get_temp_dir()        . "\n";
    echo "temp: " . getenv('temp')            . "\n";
    echo "tmp:  " . getenv('tmp')             . "\n";
?>

Documented officially: https://www.php.net/manual/en/configuration.file.php#example-36

Before I begin, I just want to specify my configurations: I'm using Windows 7-64bit, PHP 5.4.3, Apache HTTP Server 2.2.x, I've set my environmental variable PHP_HOME=C:\tools\php-5.4.3 (PHP installation directory). I use the variable in my httpd.conf and php.ini file

Note: I will be omitting some text for brevity.

In the httpd.conf file

# For PHP 5 do something like this:
LoadModule php5_module "${PHP_HOME}/php5apache2_2.dll"
AddType application/x-httpd-php .php

# configure the path to php.ini
PHPIniDir "${PHP_HOME}"

In the php.ini file

; Directory in which the loadable extensions (modules) reside.
; http://php.net/extension-dir
; extension_dir = "./"
; On windows:
extension_dir = "${PHP_HOME}/ext"