WordPress Filters Fundamentals

Ok, so you’ve read my previous articles on PHP basics and how to use WordPress action hooks. Now you want to play with the big boys and girls and try your hand at filters. Just a reminder that you are still essentially learning a new language. Be patient with yourself. It took me close to a year to really understand filters, so if I can help you learn it quicker than that I’ll consider this a successful article.

In my discussion of hooks and functions I have described the system of hooks in WordPress as being similar to a parking lot. The hooks are like spaces, the cars are like functions that can be parked anywhere and can be moved around. To expound on this metaphor, (or to beat it to death) I think a filter is similar to changing something about the car while leaving it in the same place. A filter is like a magic function box that takes a value in ( remember I talked about parameters in by Basics tutorial ) do something with it and send it back. So you have a red Porsche on the lot, send it to a filter and it might become a blue Porsche. Or a black pickup truck. Whatever. But it will still be parked in the back, 2 spots from the end.

When browsing WordPress core, themes and plugins, you will be alerted to the availability of a filter by the appearance of the apply_filters() function. In generic terms it will look like so:

It might look like:


or even:

Parameters for the Filters

But I repeat, the big tip off is the apply_filters() function. The apply_filters() function can take an unlimited number of parameters but only the first two are required.

  1. The first is the name of the filter. I was very confused early on because the Thematic framework tended to name the filters the same thing as the functions that contained them. And then in the one instance where they were different, I couldn’t get my filter to work for anything. I swore and threw things, cried and probably shaved a few days off my life from the stress. Turns out their sharing names was a convenience factor that makes a lot more sense to me now. So, just remember that first parameter is the filter’s name.

  2. This is an important one, because this parameter is the variable that your function will receive. This is the value we’re going to change. This is the red car that we will turn blue.

3+. These are other variables that might give you some useful information for manipulating the variable getting filtered. Remember we talked about scope in the PHP Basics. Instead of declaring these globally and hoping they arrive intact, WordPress will send your filtered variable along with some friends.

Introducing add_filter()

add_filter() is the function that tells WordPress that you want to attach a function to the apply_filters() statement in order to modify the variable.

add_filter() accepts 4 parameters:

  1. The name of the target filter matches the name provided in apply_filters().
  2. Your callback function can be named anything, but if the function does not exist, you’ll get a White Screen of Death.
  3. The priority, or order, that the callbacks will be fired in. The default is 10. Anything lower than 10 will run earlier, anything higher than 10 will run later.
  4. This is the number of parameters that you are passing into your callback function. One is the default. However, some filters provide access to other variables (like our fake $other_arg above), and if you want to use them in your filter function then you have write the correct number at the end of the add_filter() statement and name them inside the parentheses of your callback function.

A Filter with Training Wheels

In super generic terms let’s take a look at the basic set up for filtering.

The structure is almost exactly the same as that for add_action(). In reality, we are adding an action, just of a slightly different type: one that modifies a specific variable’s value. Since we don’t declare it, the filter is added with a default priority of 10 and the default passage of 1 variable ( ie. the variable that you are filtering ).

To complicate things a little, let’s change the priority and pass an additional variable to our callback. Passing additional variable can sometimes be helpful for conditionally altering the variable you are filtering.

If you have 2 parameters in the function, but don’t have the number 2 at the end of add_filter() you will get White Screen of Death. If you have the number 2, but don’t declare two parameters, guess what, you will also get WSOD. So make sure your parameters match up. And you absolutely, must, sans doute, return a value. If you do not, that is essentially the same as returning a null string. If you echo something here, it will likely not appear where you expect it, so don’t. RETURN, RETURN, RETURN.

Remember in my PHP basics I talked about two functions playing catch. The return statement, is you throwing your modified version of the variable back to the apply_filters() function so that WordPress can continue on its merry way, but using your value now.

You’re a Real Boy Now, Pinocchio.

A Real Filter If you’ve managed to read all this, then… well… Bless you. I want to end on a real filter that you can drop into your theme’s functions.php and actually see a working result.

WordPress has a filter called the_title() that lets you modify the title of every post! As written above it will literally add BACON everywhere the_title() is called. Bacon everywhere, all the time has its appeal, sure, but I will whet your appetite with some neato WordPress conditional logic that will only add Bacon to the titles in the main loop, and only on the front-end.


I hope that helps things make more sense. Remember, THIS IS HARD!! Do not be discouraged if you don’t get it right away. I cannot provide specific support in the comments ( Hire Me, instead! but do let me know if there are places where I could be more clear.

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