Everytime you need to use a string consider if you really need double-quoted string. If you do not want to expand variable names or use special escape characters, you are encouraged to use single-quoted strings. The reason is obvious - use of double-quoted strings is more time consuming in the runtime.
'foo bar' // good
"foo {$bar}" // good
"foo\nbar" // good
"foo bar" // bad
Variables to be expanded in double-quoted strings should be enclosed with curly braces. This helps to avoid problems with confusing constructions. For example
"{$foo}bar" // good
has completely different meaning than:
"$foobar" // bad
On the other hand, it is the only way to use array indices directly in strings:
"{$foo['bar']}" // good
"$foo['bar']" // bad