Cannot Use is_single() and is_admin in functions.php

Question

I'm using a Thematic childtheme for my wordpress website, and instead of putting all my functions into the functions.php file, I thought I make it more organised by creating multiple other function files (admin_functions.php, single_functions.php ...) and included these files in my childtheme's functions.php file as follows:

require_once ('my-functions/admin-functions.php');
require_once ('my-functions/single-functions.php');

This works perfectly fine.

However, I'm trying to make wordpress load only the needed files by using the conditional tags as follows:

if(is_admin())
   require_once ('my-functions/admin-functions.php');
elseif(is_single())
   require_once ('my-functions/single-functions.php');

and this just won't work.

I can't seem to find a reason why. I thought that maybe there are some wordpress functions that must be called first before the conditional functions (is_single and is_admin) are usable. I can't seem to get anywhere though.

Any help is most appreciated. Thanks

Answer 1

As this is an important question with no real answer, I will provide what I found after hunting a solution down, though I don't have an answer as to why it doesn't work like we all think it should.

You need to hook into WordPress somewhere, like this:

// load our posts-only PHP
add_action( "wp", "only_posts" );
function only_posts() {
    if( is_single() ) {
        // we are on a single post
        include_once( "posts.php" );
    }
}

So in your case specifically, I would suggest writing it as follows to get is_single and is_admin to work in functions.php:

add_action( "wp", "include_conditionals" );
function include_conditionals() { 
    if( is_admin() ) {
        require_once( "my-functions/admin-functions.php" );
    } else if( is_single() ) {
        require_once( "my-functions/single-functions.php" );
    }
}

See here for more information.

Answer 2

First let's go ever when and where you can use these conditionals:

• is_admin() would work pretty much universally. WP_ADMIN constant it checks for is defined extremely early in admin entry points and by exclusion if it's not there you aren't there
• is_single() is much less robust, ir relies on global $wp_query being set up, such conditionals are primarily meant to be used inside theme's template files

Second, there is not much point for conditional loading. WordPress itself does this, but frankly it's questionable win (more so with how opcode caching works in reality, which any serious site should be using) and a lot of inconvenience when you do need functions in "wrong' context.

For classes this is primarily handled with autoload. For functions however the path of least resistance is just to load them and be done with it.