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?