How to Add a Class to the First Post in the WordPress Loop

If you wanted to add emphasis to the first post in your blog adding this snippet to the functions.php of your WordPress theme will add the class of “first” to your first post.

Instructions
Add this code to your functions.php file.

<?php
   add_filter( 'post_class', 'wps_first_post_class' );
   function wps_first_post_class( $classes ) {
      global $wp_query;
      if( 0 == $wp_query->current_post )
      $classes[] = 'first';
      return $classes;
   }
?>