I have the apache2.0, PHP5.2.4 and the directive in the httpd.conf:

AddType application/x-httpd-php-source .php .php3 .php4 .php5 .php6

AddType directive is used to maps the given filename extensions onto the specified content type. This is the only meaning of this directive. But why does this method switch off PHP handler, that assigned .php extensions, and I can view source code of scripts in my browser?

Why does this method switch on PHP handler? This simply must send header "Content-Type: application/x-httpd-php5" to my browser and this must be only meaning of directive AddType from mod_mime. I'm confused.

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any other guesses –  egor Sep 15 '10 at 14:17

Your question is a bit difficult to understand. Are you stating that when you use AddType application/x-httpd-php-source .php .php3 .php4 .php5 .php6 that you can see the actual PHP code itself rather than the processed HTML, but with AddType application/x-httpd-php5 .php it works?
You should only add this directive to .phps or whatever name you have to php source files you want shown with syntax highlighting, not the ones you want interpreted. If you want the .php{3,4,5,6} to work normally, add it to the correct handler x-httpd-php. There's no way to assign the same extension to both handlers and have it work.