Something to keep in mind as you decide which quotes to use...
With double quotes, you can put variables inside them and they'll get resolved. With single quotes they won't.
$str1 = "lamb";
$str2 = "Mary had a little $str1";
echo $str2;
outputs: Mary had a little lamb
$str2 = 'Mary had a little $str1;
outputs: Mary had a little $str1