Class to add custom fields to the user profile in WordPress.

class.TTT_User_Profile_Addon.php

<?php
/**
 * Adds a form field to the edit profile page.
 *
 * Usage:
add_action( 'init', 'init_profile_addons' );
function init_profile_addons()
{
	$GLOBALS['extended_author_text'] = new TTT_User_Profile_Addon(
		array (
			'name' => 'extended_author_text'
		,	'label' => 'Second text field'
		,	'description' => '<p>This text will be shown at the top of your author archive. you may use HTML.</p>'
		,	'markup' => '<table class="form-table"><tr><th>%label%<br /><br />%description%</th>
			<td><textarea name="%name%" id="%id%" rows="10" cols="30">%content%</textarea></td></tr></table>'
		)
	);
}
 *
 * @author Thomas Scholz http://toscho.de
 * @version 1.2
 *
 */
class TTT_User_Profile_Addon
{

	public $settings = array (
	/* The name attribute. */
		'name'
	,	'label'       => ''
	,	'description' => ''

	/* You may use the following placeholder:
	 * %name%        - name attribute
	 * %label%       - label text
	 * %description% - additional text
	 * To use more placeholders, extend markup_filter().
	 */
	,	'markup'      => ''
	/* If both are not FALSE, they will replace the 'markup', and a
	 * table will be created. Uses the same placeholders as 'markup'.
	 */
	,	'th'          => FALSE
	,	'td'          => FALSE
	/* Capabilities to show and edit the field.
	 * Useful, if want to add a field that only administrators or editors
	 * may edit or view.
	 */
	,	'cap_show'    => 'read'
	,	'cap_save'    => 'edit_user'
	);

	/**
	 * Constructor
	 * @param array $args See settings. 'name' and 'markup' required.
	 */
	public function __construct( $args )
	{
		$this->settings = array_merge( $this->settings, $args );

		// The id attribute should be different to name, otherwise it doesn’t
		// work in Opera.
		empty ( $this->settings['id'] )
			and $this->settings['id'] = $this->settings['name'] . '_id';

		FALSE !== $this->settings['th'] and FALSE !== $this->settings['td']
			and	$this->settings['markup'] = '<table class="form-table"><tr><th>'
				. $this->settings['th'] . '</th><td>' . $this->settings['td']
				. '</td></tr></table>';

		add_action( 'show_user_profile',        array ( $this, 'show' ) );
		add_action( 'edit_user_profile',        array ( $this, 'show' ) );
		add_action( 'personal_options_update',  array ( $this, 'save' ) );
		add_action( 'edit_user_profile_update', array ( $this, 'save' ) );
	}

	/**
	 * Prints the form.
	 * @param  object $user
	 * @return void
	 */
	public function show( $user )
	{
		if ( ! current_user_can( $this->settings['cap_show'], $user->ID ) )
		{
			return;
		}

		$label   = "<label for='{$this->settings['id']}'>{$this->settings['label']}</label>";
		$markup  = strtr( $this->settings['markup'],
			array (
				'%name%'        => $this->settings['name']
			,	'%id%'          => $this->settings['id']
			,	'%label%'       => $label
			,	'%description%' => $this->settings['description']
			)
		);
		$old_val = trim( get_the_author_meta( $this->settings['name'], $user->ID ) );
		$markup  = $this->markup_filter( $markup, $old_val );

		print $markup;
	}

	/**
	 * Saves the data.
	 * @param  int $user_id
	 * @return void
	 */
	public function save( $user_id )
	{
		if ( ! current_user_can( $this->settings['cap_save'], $user_id ) )
		{
			return;
		}

		$input = empty ( $_POST[ $this->settings['name'] ] ) ? '' : $_POST[ $this->settings['name'] ];
		$input = $this->prepare_input( $input );

		update_user_meta( $user_id, $this->settings['name'], $input );
	}

	/**
	 * Prepares the user input. For extensions.
	 * @param  string $input
	 * @return string
	 */
	public function prepare_input( $input )
	{
		return $input;
	}

	/**
	 * Prepares the form markup
	 * @param  string $markup
	 * @param  string $old_val
	 * @return string
	 */
	public function markup_filter( $markup, $old_val )
	{
		$old_val = htmlspecialchars( $old_val, ENT_QUOTES, 'utf-8', FALSE );
		return str_replace( '%content%', $old_val, $markup );
	}
}

class.TTT_User_Profile_Checkbox.php

<?php
/**
 * Template for a simple checkbox.
 */
class TTT_User_Profile_Checkbox extends TTT_User_Profile_Addon
{
	public function prepare_input( $input )
	{	// Convert the checkbox value to integer
		return '' == trim( $input ) ? 0 : 1;
	}

	public function markup_filter( $markup, $old_value )
	{	// Preselect the checkbox if neccessary.
		$checked = 1 == $old_value ? ' checked="checked"' : '';
		return str_replace( '%checked%', $checked, $markup );
	}
}

functions.php

<?php
// Usage example for the functions.php

add_action( 'init', 'ttt_init_profile_addons' );

/**
 * Registers the extra fields for the user profile editor.
 */
function ttt_init_profile_addons()
{
	$GLOBALS['ttt_show_profile'] = new TTT_User_Profile_Checkbox(
		array (
			'name' => 'ttt_show_profile'
		,	'label' => 'Show a short profile box on my posts.'
		,	'th' => ''
		,	'td' => '<input type="checkbox" name="%name%" id="%id%" %checked% /> %label%'
		)
	);
}