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add_filter function in WordPress
February 17, 2021

We can use add_filter to hook a function or a method to a specific filter action.

WordPress has tons of functions to make changes in the website – backend and frontend. WordPress also has filter hooks.

filter hooks allow plugins to modify various types of internal data at runtime.

A plugin can modify data by binding a callback to a filter hook. When we apply the filter, each bound callback is run in order of priority.

Syntax


add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1 )

Where,

$tag is string and required. It’s the name of the filter to hook.

$function_to_add is required as well. It is callable. The callback to be run when the filter is applied. You can also pass a class method as a callback.

$priority is optional but if we want to pass it then it has to be an integer. The default value is 10.

$priority is used to specify the order in which the functions associated with a particular action are executed. Lower numbers means it will execute first. If two functions have same priorities then they will be executed in the order in which they were added to the action.

$accepted_args – as the name suggests – It’s the number of arguments the function accepts. It is optional but if we want to pass it then it has to be an integer. The default value is 1. It makes sense! The function will always accept at least one argument.

Hooked functions can take extra arguments that are set when the matching do_action() or apply_filters() call is run.

Examples

Simple callback



function square( $var ) {
    return $var*$var;
}
add_filter( 'square_filter', 'square' );



Change the frontpage sections



add_filter( 'twentytwentyone_front_page_sections', 'prefix_custom_front_page_sections' );
     
function prefix_custom_front_page_sections( $num_sections )
{
  return 6;
}



Display custom length of post excerpt



if( ! function_exists( 'prefix_custom_excerpt_length' ) )
{
    function prefix_custom_excerpt_length( $length )
    {
        return 40;
    }
}
add_filter( 'excerpt_length', 'prefix_custom_excerpt_length', 999 );



Inject a CLASS/ID CSS in content



//Add Class/ID to Post Content
add_filter('the_content', 'xai_my_class');
function xai_my_class($content)
{
    //Replace the instance with the Class/ID markup.
    $string = '<ul'; //your tag
    $replace = '<ul class="detail-list"'; //add your class/id and tag
    $content = str_replace( $string, $replace, $content );
    return $content;
}



Credit goes to: WordPress Official Docs

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