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The best way to understand what a hook does is to look at where it occurs in the source code.
do_action( "hook_name" )apply_filters( "hook_name", "what_to_filter" ).Remember, this hook may occur in more than one file. Moreover, the hook's context may change from version to version.
| Line | Code | 
|---|---|
| 4316 |      $post_id = (int) $post_id; | 
| 4317 |      if ( !$post = get_post( $post_id ) ) | 
| 4318 |           return false; | 
| 4319 |      if ( !is_array( $imagedata = wp_get_attachment_metadata( $post->ID ) ) ) | 
| 4320 |           return false; | 
| 4321 |  | 
| 4322 |      $file = get_attached_file( $post->ID ); | 
| 4323 |  | 
| 4324 |      if ( !empty($imagedata['thumb']) && ($thumbfile = str_replace(basename($file), $imagedata['thumb'], $file)) && file_exists($thumbfile) ) | 
| 4325 |           return apply_filters( 'wp_get_attachment_thumb_file', $thumbfile, $post->ID ); | 
| 4326 |      return false; | 
| 4327 | } | 
| 4328 |  | 
| 4329 | /** | 
| 4330 |  * Retrieve URL for an attachment thumbnail. | 
| 4331 |  * | 
| 4332 |  * @since 2.1.0 | 
| 4333 |  * | 
| 4334 |  * @param int $post_id Attachment ID |