How to find taxonomy name using only taxonomy TERM ID (or taxonomy term name)

I am developing a multiple search filters function and I need to filter the search results page.

I have 3 custom post types, each one with its custom taxonomy. So I have the custom post types:

1. CPT books;
2. CPT fruits;
3. CPT cars

Each one of these custom post types has a custom taxonomy attached. So we have

1. CPT books with taxonomy books-category
2. CPT fruits with taxonomy fruits-category
3. CPT cars with taxonomy cars-category

And each of these custom taxonomies have a couple of terms in them. So we have:

1. CPT books with taxonomy books-category, with terms: drama, crime, adventure
2. CPT fruits with taxonomy fruits-category with terms: apple, orange, banana
3. CPT cars with taxonomy cars-category with terms: mercedes, ford

In the searchform.php I have the search input AND a function that gets ALL the terms from ALL my custom taxonomies and puts them into a drop down.

So the HTML looks like this:

<ul> 
    <li><input type="text" value="" name="s" class="field s"></li>
    <li>Categories:</li>    
    <li>
        <select class="postform" id="cat" name="cat">
            <option selected="selected" value="0">All categories</option>
            <option value="99">Adventure</option>
            <option value="41">Apples</option>
            <option value="23">Banana</option>
            <option value="31">Crime</option>
            <option value="19">Drama</option>
            <option value="103">Ford</option>
            <option value="24">Mercedes</option>
            <option value="67">Orange</option>
        </select>
    </li>
    <li><input type="submit" id="searchsubmit" value="Search" /></li>
</ul>

When a user leaves the search input empty, but selects a taxonomy term in that drop-down, I need the search page to filter my search results so that it displays only custom post types filed under that taxonomy term.

The big mystery here how can I find a way to get the TAXONOMY NAME to use in the WP_Query class, giving the fact that I have the TERM NAME and the TERM ID (the “value” field from my drop-down form). I could do this easy in taxonomy.php, but here I am in search.php!

Any help would be greatly appreciated 🙂

Best,
Gabriela

How about using the get_term function? If you look at the code the sql it uses is as follows:

$_term = $wpdb->get_row( $wpdb->prepare( "SELECT t.*, tt.* FROM $wpdb->terms AS t INNER JOIN $wpdb->term_taxonomy AS tt ON t.term_id = tt.term_id WHERE tt.taxonomy = %s AND t.term_id = %d LIMIT 1", $taxonomy, $term) );

which should be exactly what you need!