Example of Use cases for Filters

functions.php

<?php

/**
 * code #1 - update the post excerpt character length
 * @param integer
 * @return integer
 */
function my_excerpt_length($length) 
{
	return 65; 
}
add_filter('excerpt_length', 'my_excerpt_length');



/**
 * code #2 - replace 'Read more' text with new string same URL
 * @param string
 * @return string
 */ 
function new_excerpt_more($text) 
{  
	return ' [...]'; 
 
}
add_filter('excerpt_more', 'new_excerpt_more');


/**
 * code #3 - remove automatic <p> tags in Wordpress editor
 */
remove_filter('the_content', 'wpautop');


/**
 * code #4 - append 'first' class onto the 1st <p> element in each post
 * @param string
 * @return string
 */
function first_paragraph($content){
	global $post;
 
	if ($post->post_type == "post") {
		return preg_replace('/&lt;p([^>]+)?>/', '&lt;p$1 class="first">', $content, 1);
	}
	else {
		return $content;
	}
}
add_filter('the_content', 'first_paragraph');


/**
 * code #5 - add & remove custom Wordpress social contacts from user profile
 * @param array
 * @return array
 */
function cusom_social_contact_links($contacts) {
  // Adding
  $contacts['twitter']  = 'Twitter';
  $contacts['youtube']  = 'YouTube';
  $contacts['dribbble'] = 'Dribbble';
 
  // Removing
  unset($contacts['yim']);
  unset($contacts['aim']);
  unset($contacts['jabber']);
 
  return $contacts;
}
add_filter('user_contactmethods','cusom_social_contact_links', 10, 1);


/**
 * code #6 - Remove the URL Field from the WordPress Comment Form
 * @param array
 * @return array
 */
function remove_comment_fields($fields) {
    unset($fields['url']);
    return $fields;
}
add_filter('comment_form_default_fields','remove_comment_fields');