WordPress User Contact Methods Generator

/**
 * Adds fields to the Profile screen's "Contact Info" section.
 *
 * The second parameter has a default because add_filter() is told to pass two
 * arguments: core always does, but a plugin calling
 * apply_filters( 'user_contactmethods', $methods ) with one would be an
 * ArgumentCountError on PHP 8 against a callback that requires both.
 */
function wp_skills_user_contact_methods( $methods, $user = null ) {
	$methods[ 'linkedin' ] = __( 'LinkedIn', 'text-domain' );

	return $methods;
}
add_filter( 'user_contactmethods', 'wp_skills_user_contact_methods', 10, 2 );

/*
 * No save routine, deliberately.
 *
 * WordPress saves these itself. edit_user() walks
 * wp_get_user_contact_methods() and assigns each posted value with
 * sanitize_text_field(), which is why a contact method needs no
 * personal_options_update hook, no nonce of its own, and no capability check —
 * the profile form's own checks already ran. Verified in Playground: with this
 * filter in place and nothing else, posting a field to edit_user() stored the
 * sanitized value, and a field left out of the request kept the value it had.
 *
 * Two consequences worth knowing. Core renders every contact method as a plain
 * text input, so these cannot be typed as url, email or date — the field type
 * is not yours to choose. And sanitize_text_field() strips tags but does not
 * validate, so a field meant to hold a URL can hold anything; escape it at the
 * point of output rather than trusting what is stored.
 */

// Reading the values in a template, inside the loop or with an explicit user id:
//
//   $linkedin = get_the_author_meta( 'linkedin', $user_id );
//   if ( $linkedin ) {
//       printf( '<a href="%s">%s</a>', esc_url( $linkedin ), esc_html( $linkedin ) );
//   }
//
// get_the_author_meta() returns the raw stored string, so it is escaped at the
// point of output: esc_url() for anything that becomes an href, esc_html()
// otherwise. A profile field is user input like any other.