input-checkbox.php

<?php
/**
 * Adding a settings field in the already existing settings
 * page: general.
 */
add_action( 'admin_init', function() {

	/**
	 * To create a settings field, it needs to be registered first.
	 * Here we register a settings field called 'brand-name'.
	 *
	 * This 'brand-name' will be also the key in the options table
	 * and it should also be used as the name of the form field.
	 */
	register_setting( 'general', 'brand-name' );

	/**
	 * Here we add a settings field after registering it above.
	 */
	add_settings_field(
		'brand-name',
		'Brand Name',
		'render_radio_for_brand_name',
		'general'
	);
});

/**
 * This will output the radio field.
 */
function render_radio_for_brand_name() {
	$brand_name = get_option( 'brand-name' );

	printf(
		'<p><label for="brand-name-yes"><input type="checkbox" name="brand-name[option-yes]" id="brand-name-yes" value="yes" %s/>Yes</label></p>',
		checked( $brand_name['option-yes'], 'yes', false )
	);

	printf(
		'<p><label for="brand-name-no"><input type="checkbox" name="brand-name[option-no]" id="brand-name-no" value="no" %s/>No</label></p>',
		checked( $brand_name['option-no'], 'no', false )
	);

	printf(
		'<p><label for="brand-name-maybe"><input type="checkbox" name="brand-name[option-maybe]" id="brand-name-maybe" value="maybe" %s/>Maybe</label></p>',
		checked( $brand_name['option-maybe'], 'maybe', false )
	);
}

input-radio.php

<?php
/**
 * Adding a settings field in the already existing settings
 * page: general.
 */
add_action( 'admin_init', function() {

	/**
	 * To create a settings field, it needs to be registered first.
	 * Here we register a settings field called 'brand-name'.
	 *
	 * This 'brand-name' will be also the key in the options table
	 * and it should also be used as the name of the form field.
	 */
	register_setting( 'general', 'brand-name' );

	/**
	 * Here we add a settings field after registering it above.
	 */
	add_settings_field(
		'brand-name',
		'Brand Name',
		'render_radio_for_brand_name',
		'general'
	);
});

/**
 * This will output the radio field.
 */
function render_radio_for_brand_name() {
	$brand_name = get_option( 'brand-name' );

	printf(
		'<p><label for="brand-name-yes"><input type="radio" name="brand-name" id="brand-name-yes" value="yes" %s/>Yes</label></p>',
		checked( $brand_name, 'yes', false )
	);
	printf(
		'<p><label for="brand-name-no"><input type="radio" name="brand-name" id="brand-name-no" value="no" %s/>No</label></p>',
		checked( $brand_name, 'no', false )
	);
	printf(
		'<p><label for="brand-name-maybe"><input type="radio" name="brand-name" id="brand-name-maybe" value="maybe" %s/>Maybe</label></p>',
		checked( $brand_name, 'maybe', false )
	);

}

input-text.php

<?php
/**
 * Adding a settings field in the already existing settings
 * page: general.
 */
add_action( 'admin_init', function() {

	/**
	 * To create a settings field, it needs to be registered first.
	 * Here we register a settings field called 'brand-name'.
	 *
	 * This 'brand-name' will be also the key in the options table
	 * and it should also be used as the name of the form field.
	 */
	register_setting( 'general', 'brand-name' );

	/**
	 * Here we add a settings field after registering it above.
	 */
	add_settings_field(
		'brand-name',
		'Brand Name',
		'render_text_input_for_brand_name',
		'general'
	);
});

/**
 * This will output the input field.
 */
function render_text_input_for_brand_name() {
	$brand_name = get_option( 'brand-name' );

	printf(
		'<input type="text" name="brand-name" id="brand-name" value="%s"/>',
		$brand_name
	);
}