Single quote vs Double quote in PHP

It is slightly faster to use single quotes.
This is because when you use double quotes PHP has to parse to check if there are variables in there.

So, if you do:
    $bar = 'world';
    $foo = "hello $bar";
    $baz = 'hello $bar';

$foo would contain "hello world" while $baz could contain "hello $bar"

Whenever you are not using variables inside a string it's good practice to just use single quotes so PHP doesn't have to parse the string.

--Paolo Bergantino from Stackoverflow.com