? behind a string php code example

Example 1: add more data to variable php

$a = "Hello ";
$a .= "World!";

Example 2: php concatenate variables to function

public function getAvg(string $var) {
  # A sample from a game I wrote.
  # Concatenatin on the other side is just {stuff.$var}
  # You can also concatenate multiple variables in a varaible:
  # {$varA.$varb}
  $min = $this->{$var._min};
  # Assigns to $min object property matching $var + '_min' string.
  $max = $this->{$var._max};
  # Assigns to $max object property matching $var + '_max' string.
  $avg = (($min + $max) / 2);
  return round($avg);
}