Functions.php

If I want to modify a parent’s functions.php, how can I do this in the child?

there is no generalized answer – you will need to be specific on what you want to change – pluggable functions, filter, actions…

I guess I can see why!

I want to update a function ‘register sidebar’ with two more sidebars. I’m not sure how I can just add on a new function, or modify it in my child theme.

The beginning starts like:

if ( function_exists('register_sidebar') ) {
	register_sidebar(array(
		'name' => 'Pages Sidebar',
		'description' => esc_html__('A widget area, used as a sidebar for regular pages.', 'udesign'),
		'before_widget' => '<div id="%1$s" class="widget %2$s substitute_widget_class">',
		'after_widget' => '</div>',
		'before_title' => '<h3 class="widgettitle">',
		'after_title' => '</h3>',
	));

you can simply register the (two) new sidebars in functions.php of the child theme;

example from a Twenty Twelve child theme:

function twentytwelvechild_widgets_init() {
	register_sidebar( array(
		'name' => __( 'Third Front Page Widget Area', 'twentytwelve' ),
		'id' => 'sidebar-4',
		'description' => __( 'Appears when using the optional Front Page template with a page set as Static Front Page', 'twentytwelve' ),
		'before_widget' => '<aside id="%1$s" class="widget %2$s">',
		'after_widget' => '</aside>',
		'before_title' => '<h3 class="widget-title">',
		'after_title' => '</h3>',
	) );
}
add_action( 'widgets_init', 'twentytwelvechild_widgets_init', 15 );

(the priority 15 lets this ‘sidebar’ appear after the original ones)

https://codex.www.ads-software.com/Function_Reference/register_sidebar
https://codex.www.ads-software.com/Widgetizing_Themes#Another_complete_example

Wow awesome! So I put the following but it causes a server error. Is there some small syntax error? I know my register_sidebars work. Can I simply put these new registers in a unique function in the child theme?

I think my error is with the last add_actions. Is ‘widgets_init’ almost universal to all themes?

function u-design-register-new-sidebars() {

	register_sidebar(array(
		'name' => 'PagesSidebar_HR',
		'description' => esc_html__('A widget area, used as a sidebar for the Page Template HR.', 'udesign'),
		'before_widget' => '<div id="%1$s" class="widget %2$s substitute_widget_class">',
		'after_widget' => '</div>',
		'before_title' => '<h3 class="widgettitle">',
		'after_title' => '</h3>',
	));

	register_sidebar(array(
		'name' => 'PagesSidebar_IT',
		'description' => esc_html__('A widget area, used as a sidebar for the Page Template IT.', 'udesign'),
		'before_widget' => '<div id="%1$s" class="widget %2$s substitute_widget_class">',
		'after_widget' => '</div>',
		'before_title' => '<h3 class="widgettitle">',
		'after_title' => '</h3>',
	));

}

add_action( 'widgets_init', 'u-design-register-new-sidebars()', 17 );

php function name: no hyphens, just underlines;
https://www.php.net/manual/en/functions.user-defined.php

add_action uses the function name only, not the brackets:
https://codex.www.ads-software.com/Function_Reference/add_action

add_action( 'widgets_init', 'u_design_register_new_sidebars', 17 );

you also need to correct the function name at the top of the code.

needless to say that a functions.php needs to start with <?php at the very start in the first line.

What do you mean by

you also need to correct the function name at the top of the code.

Right now I have (with the php at the top)

// Register new sidebars

function customsidebars() {

	register_sidebar(array(
		'name' => 'PagesSidebar_HR',
		'description' => esc_html__('A widget area, used as a sidebar for the Page Template HR.', 'udesign'),
		'before_widget' => '<div id="%1$s" class="widget %2$s substitute_widget_class">',
		'after_widget' => '</div>',
		'before_title' => '<h3 class="widgettitle">',
		'after_title' => '</h3>',
	));

	register_sidebar(array(
		'name' => 'PagesSidebar_IT',
		'description' => esc_html__('A widget area, used as a sidebar for the Page Template IT.', 'udesign'),
		'before_widget' => '<div id="%1$s" class="widget %2$s substitute_widget_class">',
		'after_widget' => '</div>',
		'before_title' => '<h3 class="widgettitle">',
		'after_title' => '</h3>',
	));

}

add_action( 'widgets_init', 'customsidebars', 15 );

It doesnt crash, but does not properly register the sidebars. Am I missing something?

does not properly register the sidebars.

what are the results?
do you get the two new ‘sidebar’ areas under ‘appearance – widgets’?

your last posted code works in a child of Twenty Twelve;

if you have problems, this might come from your used theme – ‘u design’?

if so, try to contact the theme’s developer for support.

Oh I see that it created a new widget instead of using the old one in the parent theme.

Thank you very much for your time and help!!