PHP how to use a variable in an array - php

Question

I have an options page in my wordpress theme, it's to select a number of categories from a custom taxonomy.
$terms_obj = get_option('shop_features')
This returns an array with $key being the category-name and $value being either 1 or empty depending if the category was checked.
I need to grab a list of the checked categories to use in an array in another function:
add_action( 'pre_get_posts', 'custom_pre_get_posts_query' );
function custom_pre_get_posts_query( $q ) {
if ( ! $q->is_main_query() ) return;
if ( ! $q->is_post_type_archive() ) return;
if ( ! is_admin() && is_shop() ) {
$q->set( 'tax_query', array(array(
'taxonomy' => 'product_cat',
'field' => 'slug',
'terms' => array( 'knives' ), // Don't display products in the knives category on the shop page
'operator' => 'NOT IN'
)));
}
remove_action( 'pre_get_posts', 'custom_pre_get_posts_query' );
}
Where I need to insert my category-names contained in a variable $terms to replace the 'terms' => array('knives') with 'terms' => array ( $terms )
Except it doesn't work by just doing that!
Here is how I've tried to accomplish that:
function custom_pre_get_posts_query( $q ) {
if ( ! $q->is_main_query() ) return;
if ( ! $q->is_post_type_archive() ) return;
if ( ! is_admin() && is_shop() ) {
$terms_obj = of_get_option( 'eco_shop_features', $default );
foreach ( $terms_obj as $slug => $checked ) { //$key ==> value
if ( $checked > 0 )
$terms .= '\'' . $slug . '\', ';
}
$terms = rtrim( $terms, ', ' );
$q->set( 'tax_query', array(array(
'taxonomy' => 'product_cat',
'field' => 'slug',
'terms' => array( "$terms" ), // Don't display products in the knives category on the shop page
'operator' => 'NOT IN'
)));
}
remove_action( 'pre_get_posts', 'custom_pre_get_posts_query' );
}
I'm stuck of how to insert just the list of category names in to the terms field so it works as an array.

Answer 1

The right way seems to be commented out by you, below should work
foreach ( $terms_obj as $slug => $checked ) { //$key ==> value
if ( $checked > 0 )
$terms[] = $slug;
}
$q->set( 'tax_query', array(array(
'taxonomy' => 'product_cat',
'field' => 'slug',
'terms' => $terms,
'operator' => 'NOT IN'
)));

Answer 2

To get your array of category names you can do :
$terms_obj = get_option('shop_features');
$category_name_array = array_keys($terms_obj, 1);
The array_keys($terms_obj, 1) function will extract in an array all the keys of the $terms_obj for witch the value match 1.
More info : http://php.net/manual/en/function.array-keys.php

Answer 3

Just prepare an array in the loop
$terms[] = $slug
And assign to term
'terms' => $terms,