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I know that how do_action works in WordPress.

function foobar_func( $atts ){
    // Action hook before any output!
    do_action( 'dr_before_get_qod_text' );

    echo "foo and bar";

}
add_shortcode( 'foobar', 'foobar_func' );

If I have a similar code, I can display "ABC" before displaying "foo and bar" using following code.

function display_abc() {
   echo "ABC";
}
add_action( 'dr_before_get_qod_text', 'display_abc');

But I can't understand how apply_filters work. Can you please give me a practical example?

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Yes the apply_filters() hook is bit confusing at first when you encounter it. I'll try my best to explain this:

first you need to know that filter hooks allow you to change data before displaying or storing data.

Lets take an example

function list_array(){
  $arr_name = ['val1', 'val2', 'val3'];
  return $arr_name;
}

as you can see above function just returns an array and consider this code was in your plugin and you want other developer to modify the default array. That's where apply_filters() hook comes in handy

function list_array(){
  $arr_name = ['val1', 'val2', 'val3'];
  $arr_name = apply_filters( 'hook_identifier', $arr_name);
  return $arr_name;
}

now the 'hook_identifier' can be used to modify the array values, like

function add_extra_val( $arr ){
  $extra_val = ['extraval1','extraval2','extraval3']; //remember you are adding elements to array
  $arr = array_merge($extra_val, $arr);
}
add_filter('hook_identifier', 'add_extra_val');

You see the add_extra_val function takes one array ( as every filter hook, ) as an argument and then extra values are added to that array and then returned.

so what exactly did apply_filters() hook did?

it is calling all the functions ( in this case 'add_extra_val' ) that have been added to the hook ( in this case 'hook_identifier' ) at that point in the code base ( in this case when $arr_name = apply_filters( 'hook_identifier', $arr_name) ).

This is it. hope you understand now.

| improve this answer | |
  • Thanks, great explanation.. Is this applies for only modify object or arrays? Or are there more things? – I am the Most Stupid Person Jul 19 '18 at 8:50
  • as I said filter hooks are used to modify data and data can be any valid php data type. I choose array for demonstration purpose. – maverick Jul 19 '18 at 8:56
  • Thanks.... Just one question..... Do you know pre_get_posts . This hook is called after the query variable object is created, but before the actual query is run. The pre_get_posts action gives developers access to the $query object by reference This should be filter as I think? but why it implement as a action – I am the Most Stupid Person Jul 19 '18 at 9:07
  • Visit: wpshout.com/practical-uses-pre_get_posts for more information. – maverick Jul 19 '18 at 9:19

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