I've just installed Memcached and I'd like to know if Memcached can cache images, js, css, font files, etc. on my server or it only works with scripting language?
Is it caching automatically or it have to be configured?
If not, how can I cache static files using PHP (exactly like variables values?)?
No, it doesn't, but there is also zero need to do this on a properly configured server: often accessed files on servers will be in the cache already / in memory buffers, and especially if they're static and you server has enough memory, will stay there for quite a while. Trying to serve them with Memcache will create MORE overhead, not less.
Your best option may be to use a caching layer like nginx for HTTP traffic (either as a proxy for apache or as the primary HTTP server). If you just want a proxy, Varnish is also a decent choice.
If you're stuck using Apache, here is a starting point for getting memory-based caching working: http://httpd.apache.org/docs/2.2/caching.html#inmemory
Also, you may want to look more into setting cache headers on your files so that multiple requests by the same users will not mean more file and network IO. This could be a bigger savings than explicitly caching things in memory as Linux will do some of that work for you.
convert it to string and save it in memcache
<?php
file_get_contents("/path/to/image.jpg");
?>
http://php.net/manual/en/book.memcached.php there is docs. You can save binary data, but efficient way to store complex data generated, like DB-results
