Why aren't PHP files used for (custom) CSS and JS?

Question

Why don\'t people make .php files for their CSS and JavaScript files?

Answer 1

It's not a bad idea, or all that uncommon, but there are disadvantages. Caching is an important consideration - you need to let browsers cache when the content is the same, but refresh when it will vary (e.g. when someone else logs in). Any query string will immediately stop some browsers caching, so you'll need some rewrite rules as well as HTTP headers.

Any processing that takes noticeable time, or requires a lock on something (e.g. session_start) will hold up the browser while it waits for the asset.

Finally, and quite importantly, mixing languages can make editing code harder - syntax highlighting and structure browsers may not cope, and overlapping syntax can lead to ugly things like multiple backslash escapes.

In javascript, it can be useful to convert some PHP data into (JSON) variables, and then proceed with static JS code. There is also a performance benefit to concatening multiple JS files ago the browser downloads them all in one go.

For CSS, there are specific languages such as Less which are more suited to the purpose. Using LessPHP (http://leafo.net/lessphp/) you can easily initialize a Less template with variables and callbacks from your PHP script.