you can do
$msg_gifted="<center><h1>It's a gift! </h1>";
or
$msg_gifted='<center><h1>It\'s a gift! </h1>';
Updated answer
You can not use the same quote inside the string without escaping the quote. So you either use different quotes surrounding the string or you escape it with \
. Simplified your code to show different possibilites which will all have the same output.
String in single '
quotes
$msg = '<h1>It\'s a gift!</h1><img src="http://path-to-image.jpg" />';
which is almost the same as tring in double "
quotes
$msg = "<h1>It's a gift!</h1><img src=\"http://path-to-image.jpg\" />";
Or string concatenation with different quotes just as you prefer:
$msg = "<h1>It's a gift!</h1>";
$msg.= '<img src="http://path-to-image.jpg" />';