Depends on what you are trying to do:
PHP: Strings - Manual[
^]
Basically, single quoted strings are that double quoted strings expand variable names and make special sequences work, and single quoted ones don't.
So this code:
echo "Hello World\n";
$var = 'Hello';
echo '$var world\n';
echo "$var world\n";
Prints this:
Hello World
$var world\nHello world