How to set and use global variables? Or why not to use them at all

UPDATE: My original question has been solved, but this is turning into a valid discussion about why not to use global variables, so I am updating the question to reflect that. The solution was <?php global $category_link_prop; echo esc_url( $category_link_prop ); ?> as @TomJNowell suggested.
UPDATE 2: I now have it doing exactly what I wanted. But I’m still using global scope and would be happy to find a better way.

I am trying to set up a whole bunch of global variables for the permalinks to categories to be used in various places in my theme. The main reason for this is for use in both the main navigation, as well as in a series of sub navigations that are chosen based on what category the current post is in. This is not a theme I will be releasing for use by others, but is built for one very specific purpose.

This is how I am currently creating them (I’ve only pasted in a few of the variables).

    function set_global_nav_var()
    global $prop;
    // Get the ID of a given category
    $category_id_prop = get_cat_ID( 'proposal' );
    // Get the URL of this category
    $category_link_prop = get_category_link( $category_id_prop );
    $prop = '<a href="' .esc_url( $category_link_prop ). '" title="Proposal">Proposal</a>';

    global $cb;
    // Get the ID of a given category
    $category_id_cb = get_cat_ID( 'calvinball' );
    // Get the URL of this category
    $category_link_cb = get_category_link( $category_id_cb );
    $cb = '<a href="' .esc_url( $category_link_cb). '" title="Calvinball">Calvinball</a>';
    add_action( 'init', 'set_global_nav_var' );

I can now do <?php global $prop; echo $prop; ?> int he 4 places that goes and get back the whole link for the code. When that changes I only need to change it in one place. I’m open to alternatives that do not involve the global scope.

Solutions Collecting From Web of "How to set and use global variables? Or why not to use them at all"

While I strongly advise against this, and it will not speed things up, your usage is incorrect.

When you try to use a global you must specify the global keyword first. You have specified it here when defining its value, but outside of that scope it needs to be redeclared as a global scope variable.

e.g. in functions.php :

function test() {
    global $hello;
    $hello = 'hello world';
add_action( 'after_theme_setup', 'test' );

In single.php, this will not work:

echo $hello;

Because $hello is undefined. This however will work:

global $hello;
echo $hello;

Of course you should do neither. WordPress already attempts to cache these things in the object cache. You will see no speed increase from doing this ( you may see a tiny speed decrease ), all you will get is additional complexity and the need to type out a lot of global declarations that aren’t necessary.

You would be better off using structured data, such as objects or dependency injection, or in your case, a set of function.

For example, here is a means of doing something similar via static variables ( still bad for the same reasons, but just a tiny bit less, and easier to type ) e.g.

function awful_function( $new_hello='' ) {
    static $hello;
    if ( !empty( $new_hello ) ) {
        $hello = $new_hello;
    return $hello;

awful_function( 'telephone' );
echo awful_function(); // prints telephone
awful_function( 'banana');
echo awful_function(); // prints banana

If you really want to save time by storing data somewhere to re-use, consider using the WP_Cache system

Don’t use global variables, as simple as that.

Why not to use globals

Because the use of globals makes it harder to maintain the software in the long term.

  • A global can be declared anywhere in the code, or nowhere at all, therefor there is no place in which you can instinctivly look at to find some comment about what the global is used for
  • While reading code you usually assume that variables are local to the function and don’t understand that changing their value in a function might have a system wide change.
  • If they don’t handle input, functions should return the same value/output when they are called with the same parameters. The use of globals in a function introduce additional parameters which are not document in the function declaration.
  • globals don’t have any specific initialization construct and therefor you can never be sure when you can access the value of the global, and you don’t get any error when trying to access the global before initialization.
  • Someone else (a plugin maybe) might use globals with the same name, ruining your code, or you ruining its depending on initialization order.

WordPress core has way way way much to much use of globals. While trying to understand how basic functions like the_content work, you suddenly realize that the $more variable is not local but global and need to search whole of the core files to understand when is it set to true.

So what can be done when trying to stop copy&pasting several lines of code instead of storing the first run result in a global? There are several approaches, functional and OOP.

The sweetener function. It is simply a wrapper/macro for saving the copy/paste

// input: $id - the category id
// returns: the foo2 value of the category
function notaglobal($id) {
  $a = foo1($id);
  $b = foo2($a);
  return $b;

The benefits are that now there is a documentation to what the former global does, and you have an obvious point for debugging when the value being returned is not the one you expect.

Once you have a sweetener it is easy to cache the result if needed (do it only if you discover that this function takes a long time to execute)

function notaglobal($id) {
  static $cache;

  if (!isset($cache)) {
    $a = foo1($id);
    $b = foo2($a);
    $cache = $b;
  return $cache;

This gives you the same behavior of a global but with the advantage of having an assured initialization every time you access it.

You can have similar patterns with OOP. I find that OOP usually doesn’t add any value in plugins and themes, but this is a different discussion

class notaglobal {
   var latestfoo2;

   __constructor($id) {
     $a = foo1($id);
     $this->latestfoo2 = foo2($a)

$v = new notaglobal($cat_id);
echo $v->latestfoo2;

This is a clumsier code, but if you have several values that you would like to precompute because they are always being used, this can be a way to go. Basically this is an object that contain all of your globals in an organized way. To avoid making an instance of this object a global (you want ont one instance otherwise you recompute the values) you might want to use a singleton pattern (some people argue it is a bad idea, YMMV)

I don’t like to access an object attribute directly, so in my code it will warpe some more

class notaglobal {
   var latestfoo2;

   __constructor() {}

   foo2($id) {  
     if (!isset($this->latestfoo2)) {    
       $a = foo1($id);
       $b = foo2($a);
       $this->latestfoo2= $b;
     return $this->latestfoo2;

$v = new notaglobal();
echo $v->foo2($cat_id);

Your question is involved with how php works.

Take $wpdb as example

$wpdb is a well-known global variable.

Do you know when it’ll be declared and assigned with values ?

Every page loaded, yep, every time you visit your wordpress site.

Similarly, you need to make sure those variables that you want to be globalized will be declared and assigned with corresponding values every page loaded.

Although I’m not a theme designer, I can tell the after_setup_theme is one time hook. it’ll only be triggered when theme activated.

If I were you, I’ll use init or other hooks. No, if I were you, I won’t use global variables at all…

I’m really not good at explaining things. So, you should pick up a book if you want to delve into PHP.

You can always use a singleton pattern via static getters.

    <li><?php echo MyGlobals::get_nav_prop( 'proposal' )[ 'html' ]; ?></li>
    <li><?php echo MyGlobals::get_nav_prop( 'calvinball', 'html' ); ?></li>


if ( ! class_exists('MyGlobals') ):

class MyGlobals {

    public $props;

    public function __construct(){
      $this->props = array (
        'proposal' => array( 'title' => 'Proposal', 'text' => 'Proposal' ),
        'calvinball' => array( 'title' => 'Calvinball', 'text' => 'Calvinball' ),

    public function get_nav_prop ( $term, $prop = false )
      $o = self::instance();
      if ( ! isset( $o->props[$term] ) ) {  return falst; }
      if ( ! isset( $o->props[$term][ 'html' ] ) ) {
          $id = get_cat_ID( $term );
          $link = esc_url ( get_category_link( $id ) );
          $title = $o->props[$term]['title'];
          $text = $o->props[$term]['text'];
          $o->props[$term]['html'] = '<a href="'.$link.'" title="'.$title.'">'.$text.'</a>';
          $o->props[$term]['link'] = $link;
          $o->props[$term]['id'] = $id;

      if($prop){ return isset($o->props[$term][$prop]) ? $o->props[$term][$prop] : null; }

      return $o->props[$term];

    // -------------------------------------

    private static $_instance;

    public static function instance(){

      if(!isset(self::$_instance)) {
        self::$_instance = new MyGlobals();
      return self::$_instance;


endif; // end MyGlobals