Where exactly does the edit_{taxonomy} hook fire?

Question

I'm at my wits end. I'm beginning a project that may involve using an action hook in WP that is not well-documented: edit_{$taxonomy}

I'm trying to determine exactly when this hook runs, so I just wrote a basic function with an echo for testing. Then, I added it to functions.php in a child theme. Invariably, the line with the echo causes a "Headers already sent" error. If I comment out the echo, it's fine. I thought it might have something to do with the order in which php files are loaded when using child themes, so I went head and pasted it into the bottom of the parent theme functions.php. Same issue.

Here's the code:

add_action('edit_expedited-status', 'category_status_email');
function category_status_email() {
echo "Testing";
}

Any thoughts or ideas appreciated.

Update

Note: Update not from author

On /wp-includes/taxonomy.php, you'll find (v3.3) on line 1586 the following filter under the context/action edit:

$value = apply_filters("edit_{$taxonomy}_{$field}", $value, $term_id);

Answer 1

^{Please have a read on how to work with filters and actions (I'm not repeating it here).}

The filter in a theme or plugin

// inside /wp-includes/taxonomy.php on line 1586
$value = apply_filters("edit_{$taxonomy}_{$field}", $value, $term_id);

// inside some theme file that runs before - best on functions.php during your themes bootstrap:
function wpse40709_edit_tax_field( $value, $term_id )
{
    // Do modifications

    return $value;
}
// Should be hooked in bootstrap, so best would be 'after_setup_theme' until 'admin_init'
add_filter( 'edit_YOUR-TAXONOMY-NAME_YOUR-FIELD-NAME', 'wpse40709_edit_tax_field', 20, 2 );

The filter in core

The filter gets triggered, when

sanitize_term_field($field, $value, $term_id, $taxonomy, $context)

runs. So the $field is whatever name your field has.

Answer 2

I think the problem is that you need to run the hook during init