PHP 5.3 has four kinds of literal strings.

single quoted

There is neither variable nor special character interpolation within single quotes. Single quotes need to be escaped with a backslash in order to be not taken for the delimiter. A backslash escapes a following backslash. A literal backslash needs to be escaped at end of string, as otherwise the delimiting single quote would be recognised as a literal single quote. Backslashes that preceede any other character besides backslash or single quote are literal.

double quoted

Backslash notation for \n, \r, \t, \v, \f, \\, \$, \".

Octal notation for chars: \[0-7]{1,3} Hex notation for chars: \x[0-9A-Fa-f]{1,2}

heredoc

   $param = 'dummy';
   $example = <<<EXAMPLE
   Variables are interpolated.
   $param
   EXAMPLE

Double quotes are literal. The backslashes before a double quote are literal. Unlike in Perl 5, the newline before the delimiter is not part of the string.

nowdoc

A heredoc with single quotes.

   $param = 'dummy';
   $example = <<<'EXAMPLE'
   Variables are not interpolated.
   $param
   EXAMPLE

Single quotes are literal. Backslashes are literal. Unlike in Perl 5, the newline before the delimiter is not part of the string.