I add on my wordpress a function to save a json file , when a post is saved on admin. Basically i get the title ( that will be a json string ) and convert it to JSON.
From that JSON i need to extract the id , to use like a name of the file created. But when i try to get the id , i have an empty value.

add_action('save_post', function($ID, $post) {
    // The $ID of post is different from ID that i have in post_title string

   // In field post_title i have the json string, that i will convert to json
    $parse_post = json_encode($post->post_title); 
    $post_decode = json_decode($parse_post);
    // Post Decode return this 
    // {"id":"9566","date":"2021-07-28T17:12:31","title":"test post" }

    // Here is the issue, i need to get the id from json in $post_decode
    $post_name = json_encode($post_decode->id);

    // Dir to save the file
    $dir = plugin_dir_path( __DIR__ );

    // Here i need to add the id inside $parse_post json string
    $file_path =  "$post_name'.json'";
    
    // Not show any result
     print_r($file_path);

    file_put_contents($dir.$file_path, $post_decode);
}, 10, 2);