User Contact Methods Generator

<?php
/**
 * User profile contact fields.
 *
 * The user_contactmethods filter is the supported way to add a field to the
 * profile screen. WordPress renders the input, saves the value as user meta under
 * the key you give it, and sanitises it with sanitize_text_field on the way in.
 *
 * That sanitising is not escaping. A value that is safe to store is not
 * necessarily safe to print, which is why the output helper below escapes per
 * field rather than trusting what came out of the database.
 */

defined( 'ABSPATH' ) || exit;

add_filter( 'user_contactmethods', 'my_plugin_contact_methods' );

/**
 * Adds the fields, and removes the ones nobody uses any more.
 *
 * @param array $methods Existing methods, keyed by meta key.
 * @return array
 */
function my_plugin_contact_methods( $methods ) {
	$methods['mastodon'] = __( 'Mastodon', 'my_plugin' );

	// Core dropped these in 3.6. A theme or an older plugin can still add them
	// back, so they are removed here rather than assumed absent.
	unset( $methods['aim'], $methods['yim'], $methods['jabber'] );

	return $methods;
}

/**
 * Prints a user's contact links.
 *
 * Escaping is per field and happens here, on output. esc_url for a URL, esc_html
 * for text, and a sanitize_email check before printing an address so a broken one
 * does not become a broken mailto link.
 *
 * @param int $user_id User ID.
 * @return void
 */
function my_plugin_contact_links( $user_id ) {
	$user_id = absint( $user_id );

	if ( ! $user_id ) {
		return;
	}

	$links = array();

	$value = get_user_meta( $user_id, 'mastodon', true );

	if ( $value ) {
		$links[] = sprintf(
			'<a class="contact-mastodon" href="%1$s" rel="me noopener">%2$s</a>',
			esc_url( $value ),
			esc_html__( 'Mastodon', 'my_plugin' )
		);
	}

	if ( ! $links ) {
		return;
	}

	printf(
		'<ul class="contact-methods"><li>%s</li></ul>',
		implode( '</li><li>', $links )
	);
}