PHP double quotes versus single quotes

[eluser]Nick Husher[/eluser]

2) "Use single-quotes for strings."
Benchmarks run against PHP 5.2 and 5.3 show that parsing double-quoted
strings with interpolation is no slower (and often faster) than single-
quoted strings using concatenation. When simple strings with no
variables in them are used, the performance is clearly better with
double-quoted strings due to implementation details in the engine. See
the benchmark posted at <http://pastie.org/523023>.
...
4) "Don't use concatenation with echo."
This is exactly the opposite of correct advice. The engine handles
multiple arguments to echo() in such a way that concatenation (or
double-quoted string interpolation) is actually much faster. See the
benchmark posted at <http://pastie.org/523020>.

From Gwynne Raskind, a PHP language developer. [source]