What does apply_filters(...) actually do in WordPress?

Question

Hello

I'm trying to understand some of the function in WordPress, but I can't get my head around what apply_filters(...) actually does.

Is someone able to clear this up for me with a few examples?

Cheers

Turgs

Answer 1

apply_filters($tag, $value) passes the 'value' argument to each of the functions 'hooked' (using add_filter) into the specified filter 'tag'. Each function performs some processing on the value and returns a modified value to be passed to the next function in the sequence.

For example, by default (in Wordpress 2.9) the the_content filter passes the value through the following sequence of functions:

• wptexturize
• convert_smilies
• convert_chars
• wpautop
• shortcode_unautop
• prepend_attachment
• do_shortcode

Answer 2

In the most basic terms, apply_filters is used to initialise a filter hook... add_filter assigns a new function to hooks that have already been created.

Answer 3

Here's what I'm gleaning, upon considering the most popular answer and additional resources:

• $tag seems to be a synonym for the name of the hook. (That's not particularly intuitive to me.)
• the_content is an example of a hook, of the "filter" type.
• the_content hook consists of multiple filters.
• Filters modify data. They basically filter the database, changing the data before the users view it.
• A common use of apply_filters(), for instance, is to apply the_content filters to $content. In this instance, double returns will convert to <p> tags, smiley faces will convert to icons, etc.
• "the_content" is a hook, while "the_content()" is a function.