$a = "some string";
results in $a having the value some string.
The result of an assignment expression is the value being assigned. Note that a single equal sign = is NOT for comparison!
$a = 3;
$b = ($a = 5);
does the following:
3 to $a.5 to $a. This expression yields value 5 as well.5) to $b.Thus: both $a and $b now have value 5.