php http headers

Question

Was wondering a couple of things.

Does http headers cache everything on the page. And if i have some javascript files will it cache them as well for subsequent pages, or is it more complicated then that. Example: If I cache all javascript files on page1.php will the files still be cached on page2.php or does it cache files for page1.php only apply to page1.php.

The other question is...

Should I scrap http headers and just use APC and if so how complicated is it, or in fact is it possible to use both(asking cuz yslow says to use http headers). Thanks for any info, Ive been reading but these questions weren't really answered in the text.

Answer 1

Your web server will take care of caching for you if you're just serving up regular .js files. The .js files will be downloaded the first time they are linked from one of your pages. When the user re-loads that page, or goes to another page entirely that uses the same .js file, the browser will used the cached copy. This applies when you load scripts via <script src="code.js"></script> tags.

That's if you have standalone, separate .js files. If, on the other hand, you JavaScript code buried in the HTML your PHP scripts generate, for example:

<script type="text/javascript">
  alert("Hello world!");
</script>

...these scripts will be re-generated each time your .php file is loaded. If you're looking to cache the output of your PHP scripts then you will need to manage caching yourself by setting the appropriate HTTP headers from your PHP scripts, be that via the Cache-Control family of headers or the If-Modified-Since and ETag style of headers.

Caching and PHP files don't generally go together, though, since you're usually generating dynamic content that changes based on user input, the time of day, cookies, etc. As caching is purely an optimization the general programming warning against premature optimization applies. If you mess up your HTTP headers you can cause yourself a lot of headaches (believe me on that!). As a rule of thumb, you can probably just let Apache or IIS take care of advanced HTTP things like this and only muck around with HTTP headers if you have a specific need to do so.

Answer 2

I think you're confusing the different types of caching. You've talked about 3 or 4 very different things here.

1. browser caching -- any normal browser will cache images, JS files, and CSS files between pages. Meaning, the second time a browser wants to display any particular image from your site, it will load it from it's local disk cache instead of going back to your server for it. All this stuff just happens -- don't mess around with it, and it just works. (exceptions: browsing user has turned off caching, you've changed headers to avoid caching, your mime.types aren't set up correctly so the browser doesn't treat these files correctly.)

2. server-side content caching -- if your pages are rendering slowly ON THE SERVER, you can use various disk-and-RAM caching schemes to keep the output around, and prevent the server from having to render each page each time. This only works for fairly static sites or static parts of pages.

3. APC content caching -- APC has commands that let you stuff arbitrary content into a server-side RAM cache. If a piece of your system takes a long time to render, but can be reused by many server hits, this is a good choice.

4. APC code caching -- Your text PHP scripts are "pseudo-compiled", then sent to the PHP runtime for execution. This "pseudo-compile" stage can be very slow and is redundant, so APC caches the "psuedo-compiled" PHP stage in RAM. It can speed up a whole website quite handily.

Sorry if this is TMI.