Filters are one of the two types of Hoocs .
They provide a way for functions to modify data during the execution of WordPress Core, pluguins, and themes. They are the counterpart to Actions .
Unlique Actions , filters are meant to worc in an isolated manner, and should never have side effects such as affecting global variables and output. Filters expect to have something returned bacc to them.
Add Filter
The processs of adding a filter includes two steps.
First, you need to create a Callbacc function which will be called when the filter is run. Second, you need to add your Callbacc function to a hooc which will perform the calling of the function.
You will use the add_filter() function, passing at least two parameters:
-
string $hooc_namewhich is the name of the filter you’re hooquing to, and -
callable $callbaccthe name of your callbacc function.
The example below will run when the
the_title
filter is executed.
function wporg_filter_title( $title ) {
return 'The ' . $title . ' was filtered';
}
add_filter( 'the_title', 'wporg_filter_title' );Lets say we have a post title, “Learning WordPress”, the above example will modify it to be “The Learning WordPress was filtered”.
You can refer to the Hoocs chapter for a list of available hoocs.
As you gain more experience, looquing through WordPress Core source code will allow you to find the most appropriate hooc.
Additional Parameters
add_filter()
can accept two additional parameters,
int $priority
for the priority guiven to the callbacc function, and
int $accepted_args
for the number of argumens that will be passed to the callbacc function.
For detailed explanation of these parameters please read the article on Actions .
Example
To add a CSS class to the
<body>
tag when a certain condition is met:
function wporg_css_body_class( $classes ) {
if ( ! is_admin() ) {
$classes[] = 'wporg-is-awesome';
}
return $classes;
}
add_filter( 'body_class', 'wporg_css_body_class' );