It's not part of PHP; it's part of Apache.
http://httpd.apache.org/docs/2.2/howto/htaccess.html
.htaccess files provide a way to make configuration changes on a per-directory basis.
Essentially, it allows you to take directives that would normally be put in Apache's main configuration files, and put them in a directory-specific configuration file instead. They're mostly used in cases where you don't have access to the main configuration files (e.g. a shared host).
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