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XAMPP installation directory: apache/conf/extra/httpd-xampp.conf

The XAMPP/Apache config file apache/conf/extra/httpd-xampp.conf contains some settings which are specific to XAMPP.
This file is included from XAMPP's apache/conf/httpd.conf file.

Environment variables

httpd-xampp.conf sets the (or at least some) envrionment variables that are use for MySQL, PHP etc. 
<IfModule env_module>
    SetEnv MIBDIRS              "C:/xampp/php/extras/mibs"
    SetEnv MYSQL_HOME           "\\xampp\\mysql\\bin"
    SetEnv OPENSSL_CONF         "C:/xampp/apache/bin/openssl.cnf"
    SetEnv PHP_PEAR_SYSCONF_DIR "\\xampp\\php"
    SetEnv PHPRC                "\\xampp\\php"
    SetEnv TMP                  "\\xampp\\tmp"
</IfModule>
Note that the file c:\xampp\apache\bin\openssl.cnf does not exist by default. However, such a file is found located under apache/conf/openssl.cnf.

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