child_of not working in wp_dropdown_categories

Question

I have wp_dropdown_categories working without setting child_of. Once I set child_of, all categories disappear. Here's my code, what should I do to enable child_of?

function dropdown(){
    $book_term=term_exists('book','category');
    $book_term_id = $book_term['term_id']; 
    $dropdown = array(
    'child_of'   => $book_term_id,
    'hide_empty' => 0
    )
?>
    wp_dropdown_categories( $dropdown );
<?php
}

Answer 1

The variable $book_term_id has not been set. You either need to pass it to your function or set it in your function.

Pass it to your function:

<?php dropdown($book_term_id); ?>

Set it in your function:

function dropdown(){
  $book_term_id = 5; // if a global variable, you can just use:  global $book_term_id;  
  $dropdown = array(
    'child_of'   => $book_term_id,
    'hide_empty' => 0
  )
  wp_dropdown_categories( $dropdown );
}

Answer 2

not sure if this is THE problem, but there are problems. your PHP (WP) function is outside of PHP:

?>
    wp_dropdown_categories( $dropdown );
<?php

so it'll print as text, not execute.

also, when you put it w/in PHP, make sure to add a semi-colon after the array's closing parenthesis:

  $dropdown = array(
    'child_of'   => $book_term_id,
    'hide_empty' => 0
  );
  wp_dropdown_categories( $dropdown );
}