Settings Page Generator

<?php
/**
 * The "My Plugin Settings" settings page.
 */

/**
 * Adds the menu entry.
 */
function my_plugin_menu() {
	add_options_page(
		__( 'My Plugin Settings', 'my-plugin' ),
		__( 'My Plugin', 'my-plugin' ),
		'manage_options',
		'my-plugin-settings',
		'my_plugin_render_page'
	);
}
add_action( 'admin_menu', 'my_plugin_menu' );

/**
 * The stored defaults, used before anything is saved.
 *
 * @return array
 */
function my_plugin_defaults() {
	return array(
		'api_key' => '',
		'enabled' => '',
	);
}

/**
 * Registers the option, the section and every field.
 */
function my_plugin_register_settings() {
	register_setting(
		'my_plugin_group',
		'my_plugin_settings',
		array(
			'type'              => 'array',
			'sanitize_callback' => 'my_plugin_sanitize',
			'default'           => my_plugin_defaults(),
			'show_in_rest'      => false,
		)
	);

	add_settings_section(
		'my_plugin_section',
		__( 'General', 'my-plugin' ),
		'my_plugin_section_intro',
		'my-plugin-settings'
	);

	add_settings_field(
		'api_key',
		__( 'API key', 'my-plugin' ),
		'my_plugin_render_field',
		'my-plugin-settings',
		'my_plugin_section',
		array(
			'key'         => 'api_key',
			'type'        => 'text',
			'label'       => __( 'API key', 'my-plugin' ),
			'description' => __( 'Found in your account under Developer.', 'my-plugin' ),
			'choices'     => array(),
			'label_for'   => 'api_key',
		)
	);

	add_settings_field(
		'enabled',
		__( 'Enable the sync', 'my-plugin' ),
		'my_plugin_render_field',
		'my-plugin-settings',
		'my_plugin_section',
		array(
			'key'         => 'enabled',
			'type'        => 'checkbox',
			'label'       => __( 'Enable the sync', 'my-plugin' ),
			'description' => '',
			'choices'     => array(),
			'label_for'   => 'enabled',
		)
	);
}
add_action( 'admin_init', 'my_plugin_register_settings' );

/**
 * Prints the text under the section heading.
 */
function my_plugin_section_intro() {
	// Nothing to say here yet.
}

/**
 * Renders one field.
 *
 * @param array $args Arguments passed by add_settings_field().
 */
function my_plugin_render_field( $args ) {
	$options = get_option( 'my_plugin_settings', my_plugin_defaults() );
	$key     = $args['key'];
	$value   = isset( $options[ $key ] ) ? $options[ $key ] : '';
	$name    = 'my_plugin_settings[' . $key . ']';

	switch ( $args['type'] ) {
		case 'textarea':
			printf(
				'<textarea class="large-text" rows="5" name="%1$s" id="%2$s">%3$s</textarea>',
				esc_attr( $name ),
				esc_attr( $key ),
				esc_textarea( $value )
			);
			break;

		case 'checkbox':
			printf(
				'<label><input type="checkbox" name="%1$s" id="%2$s" value="1"%3$s /> %4$s</label>',
				esc_attr( $name ),
				esc_attr( $key ),
				checked( $value, '1', false ),
				esc_html( $args['label'] )
			);
			break;

		case 'number':
			printf(
				'<input type="number" class="small-text" name="%1$s" id="%2$s" value="%3$s" />',
				esc_attr( $name ),
				esc_attr( $key ),
				esc_attr( $value )
			);
			break;

		case 'select':
			echo '<select name="' . esc_attr( $name ) . '" id="' . esc_attr( $key ) . '">';

			foreach ( $args['choices'] as $choice ) {
				printf(
					'<option value="%1$s"%2$s>%1$s</option>',
					esc_attr( $choice ),
					selected( $value, $choice, false )
				);
			}

			echo '</select>';
			break;

		default:
			printf(
				'<input type="text" class="regular-text" name="%1$s" id="%2$s" value="%3$s" />',
				esc_attr( $name ),
				esc_attr( $key ),
				esc_attr( $value )
			);
	}

	if ( '' !== $args['description'] ) {
		echo '<p class="description">' . esc_html( $args['description'] ) . '</p>';
	}
}

/**
 * Cleans the submitted values. Anything not declared here is dropped.
 *
 * @param mixed $input Raw values from the form.
 * @return array
 */
function my_plugin_sanitize( $input ) {
	$clean = my_plugin_defaults();

	if ( ! is_array( $input ) ) {
		return $clean;
	}

	$clean['api_key'] = isset( $input['api_key'] )
		? sanitize_text_field( $input['api_key'] )
		: '';

	$clean['enabled'] = empty( $input['enabled'] ) ? '' : '1';

	return $clean;
}

/**
 * Prints the page itself.
 */
function my_plugin_render_page() {
	// The menu capability hides the link. This is what actually stops the request.
	if ( ! current_user_can( 'manage_options' ) ) {
		wp_die( esc_html__( 'You are not allowed to change these settings.', 'my-plugin' ) );
	}

	echo '<div class="wrap">';
	echo '<h1>' . esc_html( get_admin_page_title() ) . '</h1>';
	echo '<form action="options.php" method="post">';

	settings_fields( 'my_plugin_group' );
	do_settings_sections( 'my-plugin-settings' );
	submit_button();

	echo '</form>';
	echo '</div>';
}