To be very honest, I've been developing for WordPress for quite some time (more than a year) but the idea of filters and actions still unclear to me. So recently, I decided to solve this problem once and for all. In this post, I'll share with you what I found out about WordPress' filters. Hopefully, it can make this topic clearer for you. Let's get started. Why do you want to use add_filter and apply_filters I'll put this short: If you want to modify an existing piece of data, you'll use add_filter to add a function to modify that piece of data You use apply_filters to run all functions that added using add_filter and return the data, after modified by all functions above A quick example A piece of code is worth a thousand lines of text so here is a quick example: $my_name = ;

add_filter( , ); { strtoupper($name);
}

$new_name = apply_filters( , $my_name);

dump($new_name); 'Luis' 'my_name' 'uppercase_it' function uppercase_it ($name) return 'my_name' As you can guess, $new_name is "LUIS" and it's what we see on the screen (I'm using var_dumper library here). Some quick note As you can see from the example above, the add_filter accepts two parameters. The first one is a tag and the second is a function. Adding function to a filter tag is useless if you don't call apply_filters with the same tag somewhere. When apply_filters run on a particular tag, it will call all functions that added to that tag. Multiple add_filter You can add as many function to a tag as you like. Let's add another function to my_name $my_name = ;

add_filter( , );

add_filter( , ); { strtoupper($name);
} { $name . ;
}


$new_name = apply_filters( , $my_name);

dump($new_name); 'Luis' 'my_name' 'uppercase_it' 'my_name' 'add_family_name' function uppercase_it ($name) return function add_family_name ($name) return ' Hernandez' 'my_name' As you can see, I added another function to the tag my_name. This function adds ' Hernandez' after the name. $new_name now would be: As you can see, order of add_filter matters. If I swap the order of the two filters (put the filter with 'add_family_name' above of 'uppercase_it'), $new_name would be: Multiple apply_filters Similar to add_filter, you can call apply_filters as many time as you like. Let's look at the code below: $my_name = ;

add_filter( , );

add_filter( , ); { strtoupper($name);
} { $name . ;
}


$new_name = apply_filters( , $my_name);
$new_name2 = apply_filters( , $my_name);

dump($new_name);
dump($new_name2); <?php 'Luis' 'my_name' 'add_family_name' 'my_name' 'uppercase_it' function uppercase_it ($name) return function add_family_name ($name) return ' Hernandez' 'my_name' 'my_name' You can guess that I got two identical result: Now, the place you put apply_filters matters. If I put one apply_filters after the first add_filter (but before the second add_filter) and the other apply_filters below all, the two results are different: $my_name = ;

add_filter( , );

$name_1 = apply_filters( , $my_name);

add_filter( , );

$name_2 = apply_filters( , $my_name);

dump($name_1);
dump($name_2); 'Luis' 'my_name' 'add_family_name' 'my_name' 'my_name' 'uppercase_it' 'my_name' However, the order depends on another thing called We'll discuss it later in this post. priority Dig into apply_filters Now let's take a peek inside apply_filters. It's quite a long function so I'd suggest you view the full source code . here Let's take a look at the first line: $wp_filter, $wp_current_filter; global $wp_filter is an array that stores all the data about tags and their functions. Let's take a look at the tag 'my_name' by adding the following code below: ($tag == )	{
    dump($wp_filter[$tag]);
} if 'my_name' Here is the result: As you can see $wp_filter['my_name'] is an instance of WP_Hook. Let's ignore all other details and focus on the callback property. You'll see that it's an associated array with the key (10) is the . Since I didn't specify the priority when calling add_filter, the default value is 10. WordPress use the priority to determine the order that functions are called. priority If I specify the priority as below: add_filter( , , );
add_filter( , , ); 'my_name' 'add_family_name' 12 'my_name' 'uppercase_it' 13 Then, the callback array will have two key, 12 and 13: Conclusion Hopefully the examples in this post helped you understand filters in WordPress a bit more. If you enjoy the post, don't forget to checkout my blog here where I write about WordPress & WooCommerce tips.

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