$str = 'This is a string with "double-quoted substring1", and "double-quoted substring2" inside.';
// Regular expression to match substrings within double quotes
$pattern = '/"([^"]*)"/';
// Find all matches
preg_match_all($pattern, $str, $matches);
// Extract the substrings
$substrings = $matches[1];
print_r($substrings);
/*
run:
Array
(
[0] => double-quoted substring1
[1] => double-quoted substring2
)
*/