You upload a text file named .htaccess that contains directives. The directives will be applied for the subdirectories as well. Make sure the text file is uploaded as ASCII/text mode when using FTP.
The directives are documented here: Apache 2.2 Directive Index
Each directive has a "Context" property and if it says ".htaccess" you can use that directive in an .htaccess file.
Well it doesn't require loading PHP for each request just to send a redirect. If you had a busy site you would avoid using PHP for such a trivial matter.
You just add that directive to the .htaccess file as well.
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