我可以使用add_filter（）来使用apply_filters（）吗？

I have a plugin that I am developing. In my plugin, I add social media share buttons to the single.php content area using the following code:

add_filter('the_content', 'myFunction');
function myFunction($content) {
    // modify the content stuff goes here    
    return $content
}

It works on hundreds of sites so far without a glitch. I have however found one customer whose site is not outputting the content. His site is using the following code to output the content on the single.php page:

echo apply_filters('the_content', $post->post_content);

As a result, my filter is not used, or at least it appears to not be being used because it is not adding any of my function's modifications to his content area.

Why isn't my filter being applied to his content? Can I modify something to make this work? What am I missing?

Its hard to say without more detail...but a good way to see what's going on is to output the hooks/ filters...

e.g.

function output_filters($filter) {
     global $wp_filter;

     if(!$filter)
        return;

     var_dump($wp_filter[$filter]);

} 

output_filters('the_content');