Filter post content in REST API

Question

I am trying to allow a plugin to modify the rendered content of posts prior to it being rendered for the WP API, or at least save it as a separate property on that JSON response.

For instance, imagine a plugin that changes the content of a post from

Here is some content : foo

to

Here is some content : bar

It's setup to use the the_post filter to make its modifications for the frontend. But the REST api is unaffected. I have tried using the rest_prepare_post filter but it's (a) apparently a bad practice to modify the rendered content directly, so I'd put it as an additional property? and (b) difficult to work with -- if I so much as try to assign $data->$data['content']['rendered'] to a variable I get the message that I am trying to convert a WP Rest response object to a string. Yet if I output that same variable to my error log, it works fine.

add_filter( 'rest_prepare_post', array( __CLASS__, 'beforeFilterRest' ), 0, 2 );

public static function beforeFilterRest( $data, $post ){

            $old_rendered_content = $data->$data['content']['rendered'];
            $data->$data['content']['rendered'] = preg_replace_callback(
                "/\s*<pre(?:lang=[\"']([\w-]+)[\"']|line=[\"'](\d*)[\"']|escaped=[\"'](true|false)?[\"']|highlight=[\"']((?:\d+[,-])*\d+)[\"']|src=[\"']([^\"']+)[\"']|\s)+>(.*)<\/pre>\s*/siU",
                array( __CLASS__, 'substituteToken' ),
                $old_rendered_content
            ); 
            return $data;
    }

I get an error right at the first line of that function. What am I missing?

Answer 1

It looks to me like you've written $data->$data[...] where you mean to have $data->data[...]. The "object to string" conversion error is probably due to the second $data, where PHP expects an object's property name but is getting another reference to the $data object.