It is an assignment and a concatenation all in one. Here are a few other variants:

$string .= " world.";  # Concatenate and assign. Same as...
$string = $string . " world.";

$num += 5;    # Add and assign.  Same as...
$num = $num + 5;

$num -= 5;    # Subtract and assign. Same as...
$num = $num - 5;

$num *= 5;    # Multiply and assign.  Same as...
$num = $num * 5;

$num /= 5;    # Divide and assign.  Same as...
$num = $num / 5;
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