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I feel like I'm being really stupid.
I want to make use of some functions in wp-admin/includes/file.php (like download_url), but it doesn't seem to be included, not in the dashboard/admin, or in the front end.

I've put:

if (function_exists('download_url')) {
    echo "YES!";
} else {
    echo "NO!";

in my theme's functions.php, and it always prints "NO!".

Do I need to do something to make these functions available?
I'm normally pretty expert with WP but I'm stumped by this very simple thing.

Thanks in advance for help.

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Ah, OK, so - helping myself along the way - if I run the function_exists() code on the admin_init hook, I get a YES!, so wp-admin/includes/file.php is loaded somewhere later on. Hmm...I want to use this in an AJAX function. Can I include all admin scripts somehow? – magicroundabout Mar 19 '13 at 11:06

OK, so there's a bunch of admin functions in wp-admin/includes. These are loaded at some point between the loading of the theme's functions.php and the admin_init hook. But that are ONLY loaded for admin-side pages. They are not loaded for front-end pages or AJAX calls.

If you want to use these functions in front-end pages of AJAX calls then you need to do:

require_once( ABSPATH . 'wp-admin/admin-functions.php' );

I would suggest using this sparingly as it will, inevitably, slow your site down.

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