The strict use of single quotes is preferred—not mandated—and the rule of readability over performance is to be maintained. As an example:

'Short String' . "\n"; "Short String\n";

Either form allowed where the string is short. Where the string is long the first form is required. Where the literal is to be followed by a variable instead of an escaped literal, the first form should prevail:

'Any String ' . $var; NOT "Any String $var"; NOR "Any String {$var}";

In all cases, readability is improved with increased whitespace, squishing stuff together to "save space" makes for a headache when you have to go back and maintain code.

There is never an excuse to quote a variable on its own, e.g. "$var", "{$var}", "{$class->var}", etc.