Why WordPress Core Functions Not Using function_exists()?

Question

This might be a stupid question. But I thought to ask it here.

It is really bad changing core file and I know that there are actions and filters in WordPress. But there are some limitations.

So why WordPress Core Functions Not Using function_exists()? I mean something like below example in core files.

if(!function_exists('wp_insert_post')){
    function wp_insert_post( $postarr, $wp_error = false ) {

        //Core Code

    }
}

Then give a way to run PHP file before core files. So any developer can modify the core files without changing core files.

Answer 1

Some do, a number of core functions is specifically designated pluggable and contained in pluggable.php for exactly this purpose.

It is also quite a messy approach and is often regretted it’s a thing at all.

The problems are loosely:

• the replacement works exactly once;
• letting extensions mess with core definitions can lead to very volatile results.

The problem with letting developers modify core files is often not how to allow this, but how to make it unnecessary. The latter is much more reasonable approach and basis for WP’s strong extensibility and success of its hooks API.