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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()
    {
    //proposal
    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>';

    //Calvinball
    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.

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1  
Which link does this statement echo esc_url( $category_link_prop ); displays ? What is your expected link? –  Vinod Dalvi Mar 4 '13 at 6:57
1  
Why would you not just use 'get_cat_ID( **** )' where ever you planned to use the global variable. I doubt there would be any speed advantage the way your doing it. From a readability standpoint, 'get_cat_ID( **** )' wins hands down. –  Chris Strutton Mar 4 '13 at 7:09
1  
Can you reword? I read your question and I'm still unsure of what you want to do and why you want to do it. My general advice would be to not use global variables, and not to pollute the global scope –  Tom J Nowell Mar 4 '13 at 12:59
1  
this is sounding a bit like an X/Y Problem. perhaps you should back up and explain exactly what your desired outcome is. I'm certain there's a far more elegant solution than setting a bunch of global vars to then just hardcode references to them in a nav elsewhere –  Milo Mar 5 '13 at 1:41
1  
create a function that outputs your menu based on the context you pass to it, that way you can keep all of the menu logic and associated vars encapsulated in one place. –  Milo Mar 5 '13 at 1:57

3 Answers 3

up vote 6 down vote accepted

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.

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I know it's a little nuts to use the global scope, but most, if not all of these variables will be used on every page. I'm open to better ideas. I am going to edit the question to make my intent a little clearer. BTW it works perfectly fine when I do <?php global $category_link_prop; echo esc_url( $category_link_prop ); ?> as per your suggestion. Thanks! –  JPollock Mar 5 '13 at 0:34
1  
Ah if my solution works, could you mark as accepted? Your global variables are just as fast as making the original call, you may want to try instead using functions so you don't need to type out 2 lines, better yet, a singleton, better yet, make all of that dynamic and in a template part included via get_template_part –  Tom J Nowell Mar 5 '13 at 9:59
    
Marked as accepted as its what I am doing now though I may go with one of the strategies @MarkKaplun is suggesting below. Using get_template_part() is an interesting idea, but I'm not sure I want to have a dir full of short files like that... –  JPollock Mar 6 '13 at 10:12
    
oooh no no you wouldn't want a file for each category, you'd want just the one that grabs the current category name and uses that. You shouldn't have to hardcode anything, imagine the hassle of hardcoding it all –  Tom J Nowell Mar 6 '13 at 10:45
    
Thanks a lot ! You saved my time. This is working perfectly.Accepted the answer... –  Sumith Harshan May 1 at 9:44

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);
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3  
Please, don't shout. Mind to explain why and provide some kind of citation? –  brasofilo Mar 4 '13 at 8:04
    
I think that you misunderstood the answer. If he wasn't trying to do early optimization by storing values in global variables his code would have worked. The shouting is because following basic established software development principles is something that can't be emphasized enough. People who do not understand those basic principle (available at your local google) should not spread code over the net. –  Mark Kaplun Mar 4 '13 at 12:11
    
Hi, Mark, apologies, my Comment was as short as your Answer and I've should made myself clearer: 1) IMO, bold is enough to make a point. 2) Although sometimes there's nothing more to say, I suspect of one line Answers: Is it okay to post a one-line answer, or would those be better as comments? –  brasofilo Mar 4 '13 at 12:37
1  
IMO this is an answer, people who come here form google should see that it is a bad idea to even think about using globals right away. –  Mark Kaplun Mar 4 '13 at 12:43
3  
It's not enough to say dont do X, you have to explain why or you look like you're saying it on a whim –  Tom J Nowell Mar 4 '13 at 13:00

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.

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