apply_filters for a theme's functions.php

[ofshard]

My primary intention with this suggestion is for switching a page's theme on the fly. Currently the only method I know of for this is to change the active theme before functions.php is included and then switching back sometime after, which I don't find very thread safe.

Anyway, I suggest that lines 278-283 in wp-settings.php which currently read:

if ( ! defined( 'WP_INSTALLING' ) || 'wp-activate.php' === $pagenow ) {
	if ( TEMPLATEPATH !== STYLESHEETPATH && file_exists( STYLESHEETPATH . '/functions.php' ) )
		include( STYLESHEETPATH . '/functions.php' );
	if ( file_exists( TEMPLATEPATH . '/functions.php' ) )
		include( TEMPLATEPATH . '/functions.php' );
}

be altered to implement the 'theme_functions' and 'theme_parent_functions' filters as below:

if ( !defined( 'WP_INSTALLING' ) || 'wp-activate.php' === $pagenow ){
	$path = (file_exists(STYLESHEETPATH . '/functions.php')) ? STYLESHEETPATH . '/functions.php' : '';
	$functions = apply_filters('theme_functions', $path);
	if( $functions != '' )
		include($functions);
	
	$path = (TEMPLATEPATH !== STYLESHEETPATH && file_exists(TEMPLATEPATH . '/functions.php')) ? TEMPLATEPATH . '/functions.php' : '';
	$functions = apply_filters('theme_parent_functions', $path);
	if( $functions != '' )
		include($functions);
}

Well, I believe you can alter the functions.php included by filtering 'stylesheet_directory' and 'template_directory' in the action 'setup_theme' as well (which is thread safe). The suggestion still stands though.

See also: #18298 - Deprecate those constants and use the function counterparts. Those functions have filters which would also kick in for this scenario I believe.

Incorporating that, my suggestion would look like this instead:

if ( !defined( 'WP_INSTALLING' ) || 'wp-activate.php' === $pagenow ){
	$stylesheet_dir = get_stylesheet_directory();
	$template_dir = get_template_directory();

	$path = (file_exists($stylesheet_dir . '/functions.php')) ? $stylesheet_dir . '/functions.php' : '';
	$functions = apply_filters('theme_functions', $path);
	if( $functions != '' )
		include($functions);
	
	$path = ($template_dir !== $stylesheet_dir && file_exists($template_dir . '/functions.php')) ? $template_dir . '/functions.php' : '';
	$functions = apply_filters('theme_parent_functions', $path);
	if( $functions != '' )
		include($functions);
}

Why exactly do you need to filter the Functions.php location if you can completely change the active theme on the fly using filters that already exist such as template_directory and those within get_stylesheet_directory()?

The ​Theme Switcher Plugin does exactly that.

As dd32 pointed out, it's possible to switch a theme on the fly using existing filters.