$string = 'Lorem ipsum dolor sit amet,[Hello world] consectetur adipiscing elit. Mauris ornare luctus diam sit amet mollis.';
$newString = preg_replace('/[.+?]/', '', $string);
echo $newString;

$input = "Lorem ipsum dolor sit amet,[Hello world] consectetur adipiscing elit. Mauris ornare luctus diam sit amet mollis.";
$output = preg_replace('/[.+]/', '', $input);
echo $output;

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris ornare luctus diam sit amet mollis.

$input = "Lorem ipsum dolor sit amet,[Hello world] consectetur [test] adipiscing elit. Mauris ornare luctus diam sit amet mollis.";

Lorem ipsum dolor sit amet, adipiscing elit. Mauris ornare luctus diam sit amet mollis.