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;

“$var” is evaluated

$str = <<<'LABEL'
$var
LABEL;

“$var” is taken literally

$var = 'Hello world';
 
$str = <<<"LABEL"
$var
LABEL;

“$var” would be intepreted.