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add_filter( 'admin_init', 'my_general_settings_register_fields' );

function my_general_settings_register_fields() {

  // Registering custom setting fields
	register_setting( 'general', 'book_url', 'esc_attr' );

	add_settings_section(
		'sss_settings_section', // Section ID
		"Santa's Shooting Star", // Section Title
		'sss_settings_callback', // Callback
		'general' // What Page?  This makes the section show up on the General Settings Page
	);

	/*
	 * Add setting fields
	 */
	 
	add_settings_field( 'book_url', '<label for="book_url">' . __( 'Book URL', 'book_url' ) . '</label>', 'book_url_settings_fields_html', 'general', 'sss_settings_section' );
}

function sss_settings_callback() { // Section Callback
	echo '<p>General settings related to theme</p>';
}

function book_url_settings_fields_html() { // Field Callback
	echo '<input class="regular-text code" type="url" id="book_url" name="book_url" value="' . get_option( 'book_url', '' ) . '" />';
}