title: In wordpress, where does function_exists(), look to decide whether a function exists?
body: <p><strong>Background:</strong> My original question was posted <a href="https://wordpress.stackexchange.com/q/125443/41479">here</a> on <a href="https://wordpress.stackexchange.com/">Wordpress StackExchange</a>
  but it may be more appropriate for this audience due to its relevancy to PHP.</p>
  
  <p><strong>Question:</strong> When using <code>function_exists()</code> in Wordpress theme development, where does that function look to decide whether a function exists?</p>
  
  <p>For example if I have <code>if ( function_exists( 'get_custom_header' )</code> in header.php, where will the function look for get_custom_header?</p>
  
  <p><strong>Special Note:</strong> From what I understand, the function refers to hash table that is made up of all the functions found by the parser so far, including any <code>includes</code> see <a href="https://wordpress.stackexchange.com/a/125456/41479">this</a>
  post.</p>