best way to concatenate variables in PHP code example

Example 1: 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);
}

Example 2: php concatenate two variables

$string = "the color is ";
$string .= "red";

echo $string; // gives: the color is red