At a first glance, you may think that using double or single quotes in PHP makes no difference.
However, double quotes will interpret what you enter. For example:
I declared a variable called favoriteFood. I am using double quotes, and if I type the variable I get: pizza
I am now using single quotes. I type the variable and I get: $favoriteFood
As you can see, the single quotes took what I typed literally. Whereas the double quotes interpreted the variable.