Analyzing genetic tags
I prefer to make a distinction on semantic level: there are no functions or methods, but instead there are properties and actions. Every property is an object (in real-world sense) and an action is something done on an object. A good mnemonic is probably to read () as do it!.

Consider a variable:

$document


You know by the name it's an object, since it's a noun.

$document-&gt;author


Even though author is probably a function/method, an author is an object and a noun, so it's a property and thus no parens here.

$document-&gt;send()
$document-&gt;author-&gt;write_book()


Again, send and write_book are functions/methods, but since those are actions (as designated by verbs), we write parens behind them. 

$document-&gt;author_count
$document-&gt;has_reviews


Again, a function/method, but a property of a document object, not an action, so no parens. A boolean value or an integer is strictly speaking not a real-world object, but it's more like an object than an action.