What is the proper way to use include with or without brackets in php
The include 'sample.php'; or require_once 'classes/DB.php'; is a preferred way.
Is preferred, it will prevent your peers from giving you a hard time and a trivial conversation about what require really is.
Side Notes:
1. require/include not a function they are language construct same as echo. Credits: @Rahil comment.
2. Also it will save time of pressing two keystroke ( & ) for a lazy developers like us :p
include and the rest are not functions, they do not need the parentheses. Parentheses are also used for grouping, for example 1 + (2 * 3). You can basically add as many parentheses around any expression as you want; 1 + (2 * 3) is equivalent to (1 + (2 * 3)) is equivalent to 1 + ((2 * 3)) is equivalent to ((1) + (((2) * (3)))).
So all include ('file.php') is doing is add unnecessary grouping parentheses around the expression 'file.php'. You may as well write include (((('file.php')))), it has exactly the same effect, which is none. It's usually only done by people who do not understand this fact and believe the parentheses to be necessary "like with other functions", or maybe who like the style.