.=

[Pixel-Artist]

what is .= used for?

[bokeh]

// this
$a .= $b;

// is the same as
$a = $a . $b

[aaronbdavis]

$foo = "bar";
$foo = $foo . "x"; // $foo is now "barx"
//is the same as
$foo .= "y"; // $foo is now "barxy"
other operators work this way also, e.g.
$int += 4; // same as $int = $int + 4;
$int -= 2; // same as $int = $int - 2;