No, single quotes are not used if there are doubles in the string.
Single quotes are used if there is no VARIABLES in the string that need to be replaced. Double quotes are used when there ARE VARIABLES in the string that need to be replaced.
Simply put:
PHP Code:
$Name = 'Adrian';
Print 'Hello $Name'; // Hello $Name
Print "Hello $Name"; // Hello Adrian
As for single quotes used in the key and double quotes used for the name-string, well thats just down to the author. Technically it will work (and you can use double quotes in the key too) but using double quotes where they are not needed wastes CPU resources. If you have a script that is 2-3000 lines long and you're using double quotes all the way through when you could just use single quotes, thats going to take more time for the script to run and finish.
For the double quotes in a key that I mentioned..
PHP Code:
$Variable = 'Item';
$Array["$Variable"] = 'Apple'; //Will work but not needed because..
$Array[$Variable] = 'Apple'; // Better.. BUT what if we need a second word in there (or an underscore)?? ..
$Array["$Variable Type"] = 'Apple'; // OR..
$Array["{$Variable}_Type"] = 'Apple'; // OR..
$Array[$Variable .'_Type'] = 'Apple';
In reality there are many ways to handle strings and variables but its just a case of picking the one that is best for the situation. I ALWAYS use single quotes for anything that has no variables inside it.
As mentioned above, "Kevin" is not wrong but it will force PHP to examine the string to see if it needs to work its magic and replace any variables. As there is not a variable in there then single quotes would be better.