How to Override a Function attached to a Filter

These days I am experimenting more and more with building extendible WordPress plugins.

Today I’ll share with you a quick tip on how to override a function that is attached to a filter hook.

Hooks are the way we build extensibility into plugins. So let’s consider an example.

In this example, I wrote a simple function that appends some text at the end of a post or page.

[php]
// Function to display some text at the end of a post

function hello_text( $content ) {
$default = get_hello_text();
return $content = $content . $default;
}

add_filter( ‘the_content’, ‘hello_text’);
[/php]

As you can see the function above gets its text from a function named get_hello_text(). It then appends it to the $content variable, containing the content of a post or page.

So lets take a look at that get_hello_text() function next.

[php]
// Function to define the text – default

function get_hello_text() {
$hello_text = ‘Hello Malta’;
return apply_filters( ‘custom_hello_text_filter’, $hello_text );
}
[/php]

The function above, get_hello_text(), defines a variable $hello_text with the words ‘Hello Malta’. Notice the very important next line however, which makes this function extendible.

We are using apply_filters, and thus creating a filter hook named custom_hello_text_filter.

Our output at this stage is, as you might guess, ‘Hello Malta‘.

At this point we can start extending our code. Take a look at what I did next

[php]
// Function to define the text – first extension

function custom_hello_text( $custom_hello_text ) {
$custom_hello_text = ‘Hello Europe’;
return $custom_hello_text;
}

add_filter( ‘custom_hello_text_filter’,’custom_hello_text’);
[/php]

What happened here? We used the add_filter function to attach the custom_hello_text() function to the filter named custom_hello_text_filter, thus this text will be used now, and the output will be ‘Hello Europe‘.

Let’s say I create another plugin and I want to extend things further, having another function which changes the text again, and we want to ignore any previous usage of the filter named custom_hello_text_filter.

Filters come with a priority parameter, the default of which is 10, so to override a function all we need to do is attach it to the filter hook with a higher priority, as follows:

[php]
// Function to define the text – second extension

function custom_hello_text_overridden( $custom_hello_text ) {
$custom_hello_text = ‘Hello World’;
return $custom_hello_text;
}

add_filter( ‘custom_hello_text_filter’,’custom_hello_text_overridden’,99);
[/php]

The higher priority of 99 will ensure that the final output will now be ‘Hello World‘.

Hope you found that useful, any improvement suggestions or questions are welcome.

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