changing variable through filters or action hooks

Question

it appears i have a problem with understanding how add_action,add_filter and apply_filters work.

somewhat abbreviated, i have the following scenario:

in a template file i have this

while ( $the_query->have_posts() ) : $the_query->the_post();
    /*get meta data for this post**/
    $meta=get_post_meta(get_the_ID(), $post_type, true );

    /***
        at this stage - amongst other things - print_r($meta) returns
        Array ( [prices] => Array ( [0] => 8 ) )
    ***/

    /**add add_action hook to change $meta (if possible)**/
    do_action('alter_loop',$meta);

    /***
        AFTER executing this action I would like to have print_r($meta) return
        Array ( [prices] => Array ( [0] => 5 ) )
        but it still returns
        Array ( [prices] => Array ( [0] => 8 ) )
    ***/

    /* more things*/

endwhile;

in a class i have the following:

function __construct() {
    add_action('alter_loop', array( $this, 'alter_loop_meta'),10,1);
}


function alter_loop_meta($meta){
    add_filter('some_identifier',array($this,'my_filter'),10,1);
    $meta = apply_filters('some_identifier',$meta);
    /***
        at this stage print_r($meta) DOES actually return
        Array ( [prices] => Array ( [0] => 8 ) )
    ***/

    return $meta;
}
function my_filter( $meta ) {
    $meta['prices'][0]='8';

    return $meta;
}

if I use

global $meta; 

in the template file it works (and i can save myself all this filtering as i can just set $meta['prices'][0]='8'; in alter_loop_meta).

however, i am wondering if there's a way to achieve this without any global variables. any hints / links etc much appreciated and - of course - happy to provide more code if necessary ...

Answer 1

It won't work the way you are doing it, but you are close. The reason you have to use a global is that you aren't setting your $meta variable with the information returned from the filter.

This:

do_action('alter_loop',$meta);

Should be this:

$meta = apply_filters('alter_loop',$meta);

Note: "Actions" do not return values. "Actions" do things. "Filters" accept data and return it.

Related:

https://wordpress.stackexchange.com/a/103644/21376
https://wordpress.stackexchange.com/a/1008/21376