Shared functionality in plugins and themes

Actions & Filters

The imho best way is to use an action to bring plugin functions into themes.

Example #1

Here’s a little plugin to test this.

<?php 
/** Plugin Name: (#68117) Print Hello! */
function wpse68117_print_hello()
{
    echo "Hello World!";
}
add_action( 'wpse68117_say', 'wpse68117_print_hello' );

Inside the theme:

<?php
/** Template Name: Test »Print Hello!« Plugin */
get_header();
// Now we call the plugins hook
do_action( 'wpse68117_say' );

What now happens / The kool kid

This way we don’t have to check the existence of a function, a file, a class, a method or maybe even a (don’t do this!) global $variable. The WP intern global already takes this for us: It checks if the hook name is the current filter and attaches it. If it doesn’t exists, nothing happens.

Example #2

With our next plugin, we’re attaching a callback function that takes one argument.

<?php 
/** Plugin Name: (#68117) Print Thing! */
function wpse68117_print_thing_cb( $thing )
{
    return "Hello {$thing}!";
}
add_filter( 'wpse68117_say_thing', 'wpse68117_print_thing_cb' );

Inside the theme:

<?php
/** Template Name: Test »Print Thing!« Plugin */
get_header();
// Now we call the plugins hook
echo apply_filter( 'wpse68117_say_thing', 'World' );

This time, we offer the user/developer the possibility to add an argument. He can either echo/print the output, or even process it further (in case you got an array in return).

Example #3

With the third plugin, we’re attaching a callback function that takes two arguments.

<?php 
/** Plugin Name: (#68117) Print Alot! */
function wpse68117_alot_cb( $thing, $belongs="is mine" )
{
    return "Hello! The {$thing} {$belongs}";
}
add_filter( 'wpse68117_grab_it', 'wpse68117_alot_cb' );

Inside the theme:

<?php
/** Template Name: Test »Print Alot!« Plugin */
get_header();
// Now we call the plugins hook
$string_arr = implode(
     " "
    ,apply_filter( 'wpse68117_grab_it', 'World', 'is yours' )
);
foreach ( $string_arr as $part )
{
     // Highlight the $thing
     if ( strstr( 'World', $part ) 
     {
         echo "<mark>{$part} </mark>";
         continue;
     }
     echo "{$part} ";
}

This plugin now allows us to insert two arguments. We can save it into a $variable and further process it.

Conclusion

With using filters and actions you’re giving better performance by avoiding unnecessary checks (compare speed of function_*/class_*/method_*/file_exists or the search for a global with in_array() for ~1k(?) filter searches). You also avoid having all those unnecessary Notices for not set variables, etc., as the plugin cares about this.