Any time you put something in “double” quotes, you are asking PHP interpreter to check that content for a variable. So even though the line do not contain variables within the double quotes, PHP will waste precious computing time scanning them anyway.
$sql = 'select * from employee';
will be much faster than
$sql = "select * from employee";
While this might seem like a big deal, it actually falls under the realm of micro-optimization – and, in most cases, the performance you gain is trivial, and not going to be noticed by any human users.
Perl happens to do this too, but it converts double-quoted strings without variables inside them into single-quoted at compile time – see this writeup(http://perlmonks.org/?node_id=577891) for more info – although I can’t tell you for sure whether PHP does the same.