That is basicly the power of the double quotes:
PHP Code:
$a = 'something';

$b1 = "a=$a"; // a=something
$b2 = 'a=$a'; // a=$a

$c['key']='value';

$d1 = "c=$c[key]"; // c=value
$d2 = "c={$c['key']}"; // c=value
$d3 = 'c=$c[key]'; // c=$c[key] 
Double qoutes expand variables, they are very usefull to make code more readable. There is no significant difference in performance.