PHP variable within variable

I have this variable: $somecontent

I want to put the value of a bunch of other variables into it…

I tried

$somecontent = '$_POST[‘opponent’] $_POST[‘type’] $_POST[‘map’] $_POST[‘winlose’]
$_POST[‘yourscore’] $_POST[‘theirscore’] $_POST[‘mvp’]
';

But that just outputted (when echoed)

$_POST[‘opponent’] $_POST[‘type’] $_POST[‘map’] $_POST[‘winlose’]
$_POST[‘yourscore’] $_POST[‘theirscore’] $_POST[‘mvp’]

How come?