WordPress: Template tag to allow for easily filtered CSS classes across templates. (http://www.blazersix.com/blog/wordpress-class-template-tag/)

html-class.php

<?php
/**
 * WordPress template tag to allow for CSS classes to be easily filtered across templates.
 *
 * @author Brady Vercher (twitter.com/bradyvercher)
 * @link http://www.blazersix.com/blog/wordpress-class-template-tag/
 *
 * @param string $id Element identifier.
 * @param array|string $classes Optional. List of default classes as an array or space-separated string.
 * @param array|string $args Optional. Override defaults.
 * @return null|array Null when displaying, otherwise an array of classes.
 */
function blazersix_class( $id, $classes = array(), $args = array() ) {
	$id = sanitize_key( $id );
	
	$args = wp_parse_args( (array) $args, array(
		'echo'    => true,
		'post_id' => null
	) );
	
	if ( ! empty( $classes ) && ! is_array( $classes ) ) {
		// Split a string.
		$classes = preg_split( '#\s+#', $classes );
	} elseif ( empty( $classes ) ) {
		// If the function call didn't pass any classes, use the id as a default class.
		// Otherwise, the calling function can pass the id as a class along with any others.
		$classes = array( $id );
	}
	
	// Add support for the body element.
	if ( 'body' == $id ) {
		$classes = array_merge( get_body_class(), $classes );
	}
	
	// Add support for post classes.
	if ( 'post' == $id ) {
		$classes = array_merge( get_post_class( '', $args['post_id'] ), $classes );
	}
	
	// A page template should set modifier classes all at once in the form of an array.
	$class_mods = apply_filters( 'blazersix_class', array(), $id, $args );
	
	if ( ! empty( $class_mods ) && isset( $class_mods[ $id ] ) ) {
		$mods = $class_mods[ $id ];
		
		// Split a string.
		if ( ! is_array( $mods ) ) {
			$mods = preg_split( '#\s+#', $mods );
		}
		
		foreach( $mods as $key => $mod ) {
			// If the class starts with a double minus, remove it from both arrays.
			if ( 0 === strpos( $mod, '--' ) ) {
				$unset_class = substr( $mod, 2 );
				unset( $mods[ $key ] );
				unset( $classes[ array_search( $unset_class, $classes ) ] );
			}
		}
		
		$classes = array_merge( $classes, $mods );
	}
	
	// Last chance to modify.
	$classes = apply_filters( 'blazersix_classes', $classes, $id, $args );
	$classes = apply_filters( 'blazersix_classes-' . $id, $classes, $args );
	
	if ( $args['echo'] ) {
		echo 'class="' . join( ' ', array_map( 'sanitize_html_class', $classes ) ) . '"';
	} else {
		return esc_attr( $classes );
	}
}