Post.php – Conditional statements for new post and edit post

Question

edit.php is the post/page list, the post creation/editing screen are 2 separate files: post-new.php and post.php.

so you can target using "load-$page" hook, e.g.:

add_action( 'load-post.php', function() {
  // add metabox or whatever you want when EDITING post
  add_action( 'add_meta_boxes', 'myplugin_meta_box_editing' );
} );

add_action( 'load-post-new.php', function() {
  // add metabox or whatever you want when creating NEW post
  add_action( 'add_meta_boxes', 'myplugin_meta_box_new' );
} );

To get more specific information (e.g. the post type being cretaed/edited) as @KrzysiekDróżdż alredy said, you can use the current screen object, however I suggest you to use get_current_screen() function for the porpose.

add_action( 'load-post.php', function() {
  // add metabox or whatever you want when EDITING post
  $screen = get_current_screen();
  // this should give you an idea of the infromation you can get
  // DO NOT USE IN PRODUCTION
  echo '<pre>';
  print_r( $screen );
  die();
} );

When adding a metabox, as alterantive or in addiction to previous methods, the callback you use for to output the metabox (the 3rd param of add_metabox function) receive the post id, so using get_post() you can retrieve the post object being edited an look at the post_status you can know if the post is just created or not:

add_action( 'add_meta_boxes', 'myplugin_add_meta_box' );

function myplugin_add_meta_box() {
  // do you need information about screen?
  // this can be used, e.g. to use different metabox callback if
  // editing or creating a new post
  // $screen = get_current_screen(); 
  add_meta_box(
    'myplugin_sectionid', // metabox id
     __( 'My Post Section Title', 'myplugin_textdomain' ), // metabox label
     'myplugin_meta_box_callback', // metabox callback, see below
     'post' // this metabox is for 'post' post type
  );
}

function myplugin_meta_box_callback( $post_id ) {
   $post = get_post( $post_id );
   $status = $post->post_status;
   if ( $status === 'new' || $status === 'auto-draft' ) {
     // this is a new post
   } else {
     // editing an already saved post
   }
}

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