This page provides info about the difference between single quotes and double quotes in PHP
Double quotes:
The test variable is Hello
Single quotes:
The test variable is $test_variable
Single quotes will display 'as-is' for the most part, whereas double quotes will be interpreted.
One clear example of this is with variables. In the above example, the double quotes will allow whatever
the value of the variable is to be included in the string. In the single quotes, it will not be recognized
as a variable, it will just be printed as another part of the string.