filter-demo.php

<?php

// Array with filter hook.
$my_array = apply_filters( 'filter_my_array', array(
  'value1',
  'value2',
  'value3',
) );

/**
 * Filter function
 *
 * @param array $array The input array.
 * @return array The filtered array.
 */
function filter_array( $array ) {
  $array[] = 'value4'; // Add another item to the array.

  return $array; // Return the array with the new item.
}

// Attaching our function to the filter hook.
add_filter( 'filter_my_array', 'filter_array' );


// OR use an anonymous function (this does the same thing)
add_filter( 'filter_my_array', function ( $array ) {
  $array[] = 'value4'; // Add another item to the array.

  return $array; // Return the array with the new item.
} );