Change Content for Specific Page

Question

I want to be able to replace a page's content if it is a very specific page.

Here is what I've written so far:

function signup () {
    global $post;
    $slug = $post->post_name;

    if (is_page( ) && strcmp($slug, 'signup_slug') == 1) {
        $content = "New Text";
    } else {
        // This is literally just "the_content()" except returning the value, not echo
        $content = $post->post_content;
        $content = apply_filters( 'the_content', $content );
        $content = str_replace( ']]>', ']]>', $content );
    }
    return $content;
}

add_action('the_content', 'signup');

When I run this code on the page named "signup_slug", it works fine. Everywhere else, the "apply_filters" gives me an error that says something about an overflow error in functions.php and that it ran 100 times and then aborted.

If I remove the "apply_filters" bit, it acts kind of funky, making every post on the front page a minimum height that is sometimes larger than normal. I think it's ignoring my "Read More" tags. Plus, all of my YouTube links show up as a URL instead of an embedded video, like they normally do.

Basically, I was wondering 2 things:

1. Is there another hook I can use that runs when you load a page, not just "the_content"? Or,
2. Is there a way to display the contents of "the_content()" without all of the funky side effects that I've been having?

Or, an even better question might be, "What's the best way to create a hook that replaces a page's content?"

Thanks!

Answer 1

Your function is signup but you're hooking check_for_signup. You're also trying to apply the_content filters from within a function that's hooked to the_content:

function wpse_225562_replace_for_signup( $content ) {
    if ( strcmp( 'signup_slug', get_post_field( 'post_name' ) ) === 0 ) {
        $content = 'Sign up, bitch.';
    }

    return $content;
}

add_filter( 'the_content', 'wpse_225562_replace_for_signup' );

Here we are simply hooking to the_content and replacing the passed-through $content with something else if the current page is signup_slug - hope that makes sense?