"foo" . '_' . 123
$abc . '_' . 123
"foo" . '_' . $num

##</code><code>##

"$abc . _ . 123"
"foo . _ . $num"

##</code><code>##

"${abc}_$_"            # 1
$abc . '_' . $_        # 2
join('_', $abc, $_)    # 3
"$abc\_.$_"            # 4
$abc . "_$_"           # 5