Strings should be quoted with single quotes if they contain no variables or control characters, otherwise use double quotes.
$flat_string = 'Hello, this is a string';
$dynamic_one = "Hello,\nthis is a string.";
$dynamic_two = "Hello $username,\n this is a string.";
The choice between using string evaluations or string additions is yours to make, depending upon the circumstances.
$string_one = "The time is $timenow and the date is $datenow.";
$string_two = 'The time is ' . time() . ' and the date is ' . $datenow . '.';
SQL Queries should always be double-quoted.
$results = $DB_site->query("SELECT field FROM " . TABLE_PREFIX . "table AS table");Variables should not be quoted if they do not need to be.
$var = "$x";
$result = strtoupper("$string");
$var = $x;
$result = strtoupper($string);
$string = fetch_something("something$var");Eval() calls should be single-quoted if possible, to avoid unncessary character escaping.
eval("\$template = \"" . fetch_template('template') . "\";");
eval('$template = "' . fetch_template('template') . '";');