[WordPress] Determine if the current post type is in a list of selected, supported post types.

acme-add-post-type-menu.php

<?php

add_action( 'admin_menu', 'acme_add_post_types_menu' );
/**
 * Adds a new submenu item to the Settings Menu that displays the list of post types supported
 * by the current theme.
 *
 */
function acme_add_post_types_menu() {

	add_options_page(
		'Current Post Types',
		'Current Post Types',
		'manage_options',
		'acme-post-types',
		'acme_options_page'
	);

}

acme-add-post-types-setting.php

<?php

add_action( 'admin_init', 'acme_add_post_types' );
/**
 * Defines the section, settings, and fields used to render the post types
 * supported by this theme.
 *
 */
function acme_add_post_types() {

	add_settings_section(
		'acme-post-types',
		'Post Types',
		'acme_display_post_types' ,
		'acme-post-types'
	);

	add_settings_field(
		'acme-post-types',
		'Available Post Types',
		'acme_render_post_types',
		'__return_null',
		'acme-post-types'
	);

	register_setting(
		'acme-post-settings',
		'acme-post-types'
	);

}

acme-display-post-types.php

<?php

/**
 * Displays a list of checkboxes each of which corresponds to the post types
 * supported by this this.
 *
 */
function acme_display_post_types() {

	$options = get_option( 'acme-post-types', array() );
	$post_types = get_post_types( array(), 'objects' );

	$html = "<ul>";
	foreach ( $post_types as $post_type ) {

		$post_type_label = $post_type->labels->singular_name;
		$current_value = isset( $options[ $post_type_label ] ) ? $options[ $post_type_label ] :  '';
		$html .= "<li>";
			$html .= "<label for='acme-post-types'>";
				$html .= "<input type='checkbox' id='acme-post-types' name='acme-post-types[$post_type_label]' value='" . esc_attr( $post_type_label ) . "'"  . checked( $current_value, $post_type_label, FALSE ) . " />";
				$html .= esc_html( $post_type_label );
			$html .= "</label>";
		$html .= "</li>";
	}
	$html .= "</ul>";

	echo $html;

}

acme-is-supported-post-type.php

<?php

add_action( 'the_content', 'acme_is_supported_post_type' );
/**
 * Determines if the current post type has been selected in the admin and, if so,
 * will append a string to the footer of the content.
 *
 * @param    string    The content for the current post type.
 * @return   string    The content that's adjusted (or not) based on the selected settings.
 */
function acme_is_supported_post_type( $content ) {

	$post_types = get_option( 'acme-post-types', array() );
	if ( in_array( ucwords( get_post_type() ), $post_types ) ) {
		$content .= '[ This post type has been selected. ]';
	}

	return $content;

}

acme-options-page.php

<?php

/**
 * Renders the sections and fields as defined using the Settings API.
 *
 * This is what allows users to check (or uncheck) the post types that
 * they want supported in the functionality demonstrated in this code.
 *
 */
function acme_options_page() {
?>
	<div class="wrap">
		<h2>Post Types</h2>
		<form action="options.php" method="post">
			<?php do_settings_sections( 'acme-post-types' ); ?>
			<?php settings_fields( 'acme-post-settings' ); ?>
			<?php submit_button(); ?>
		</form>
	</div>
<?php
}