child-theme-settings-migrator.php

<?php

/**
 * Plugin Name: Child Theme Settings Migrator
 */

add_action( 'admin_init', 'child_theme_settings_migrator' );
function child_theme_settings_migrator() {
	$theme = wp_get_theme();

	// Cap check
	if ( ! current_user_can( 'edit_theme_options' ) ) {
		return;
	}

	// If the current theme is not a child theme, bail.
	if ( $theme->get_stylesheet() === $theme->get_template() ) {
		return;
	}

	// The form that does the migration...
	add_action( 'tool_box', 'child_theme_settings_migrator_tool_box' );

	if ( isset( $_POST['action'] ) && 'migrate-child-theme-settings' === $_POST['action'] ) {
		check_admin_referer( 'migrate-child-theme-settings_' . $theme->get_stylesheet() );

		$parent_theme_mods = get_option( 'theme_mods_' . $theme->get_template() );
		$child_theme_mods  = get_option( 'theme_mods_' . $theme->get_stylesheet() );

		// Later values replace previous values -- will not overwrite any already set for child.
		$new_theme_mods = array_merge( $parent_theme_mods, $child_theme_mods );

		update_option( 'theme_mods_' . $theme->get_stylesheet(), $new_theme_mods );

		wp_safe_redirect( add_query_arg( 'migrate-success', 'true', wp_get_referer() ) );
	}
}

function child_theme_settings_migrator_tool_box() {
	$theme = wp_get_theme();
	?>
	<div class="card">
		<h2 class="title"><?php esc_html_e( 'Migrate Parent Theme Settings to Child Theme' ); ?></h2>
		<p><?php esc_html_e( 'This operation will migrate all theme mods from the parent theme to your current theme.  It is non-destructive, and will skip any settings that have already been generated by the child theme.' ); ?></p>

		<?php if ( isset( $_GET['migrate-success'] ) && 'true' === $_GET['migrate-success'] ) : ?>
			<p><strong><?php esc_html_e( 'Done!' ); ?></strong></p>
		<?php else : ?>
			<form action="<?php echo esc_url( admin_url( 'admin-post.php' ) ); ?>" method="POST">
				<input type="hidden" name="action" value="migrate-child-theme-settings" />
				<?php wp_nonce_field( 'migrate-child-theme-settings_' . $theme->get_stylesheet() ); ?>
				<?php submit_button( __( 'Migrate Settings' ) ); ?>
			</form>
		<?php endif; ?>
	</div>
	<?php
}