WordPressで独自の設定項目を設定に表示する

functions.php

function sitename_add_option_field() {
	add_settings_field(
		'sitename_home_news_posts_count',
		'ホームページ新着表示数',
		function () {
			$setting = get_option( 'sitename_home_news_posts_count' ) ?: 4;
			echo '<input name="sitename_home_news_posts_count" id="sitename_home_news_posts_count" type="number" min="3" max="8" step="1" value="'. esc_attr( $setting ) . '" class="small-text">';
		},
		'reading',
		'default',
		array(
			'label_for' => 'sitename_home_news_posts_count',
		)
	);
	register_setting( 'reading', 'sitename_home_news_posts_count', 'intval' );
}

add_filter( 'admin_init', 'sitename_add_option_field' );