In 5_3 we introduced NOWDOC in and it would be logical to allow a double quoted syntax sister of NOWDOC which acts as HEREDOC. The reason to do this is mainly consistency with with variable declarations:
$var = '$var'; // Literally $var = "$var"; // Intepreted
$var = 'Hello world'; $str = <<<LABEL $var LABEL; echo $str; // gives: Hello world
“$var” is interpreted.
$var = 'Hello world'; $str = <<<'LABEL' $var LABEL; echo $str; // gives: $var
“$var” is taken literally.
$var = 'Hello world'; $str = <<<"LABEL" $var LABEL; echo $str; // gives: Hello world
“$var” would be interpreted.