Variables in Php MCQ

< ?php 
one = 1;
two = 2;
three = 3;
four = 4;
echo "one / two + three / four";
?>

< ?php 
$on$e = 1;
$tw$o = 2;
$thre$e = 3;
$fou$r = 4;
echo "$on$e / $tw$o + $thre$e / $fou$r" ; 
?> 

< ?php 
$on_e = 1;
$tw_o = 2;
$thre_e = 3;
$fou_r = 4;
echo "$on_e / $tw_o + $thre_e / $fou_r" ; 
?>

< ?php 
$On_e = 1;
$tw_o = 2;
$thre_e = 3;
$fou_r = 4;
echo "$on_e / $tw_o + $thre_e / $fou_r";
?>

< ?php 
echo $red ;
?>

< ?php 
$hello = "Hello World";
$bye = "Bye";
echo $hello;"$bye";
?>

< ?php 
var $one = 1;
var $two = 2;
echo "$one / $two * $one / $two * $two";
?>

< ?php 
int $one = 1;
echo "$one";
?>

< ?php 
$4four = 4;
$3three = 3;
$2two = 2;
echo "$4four + $3three / $2two - 1";
?>  

< ?php 
echo "$four4 + $three3 / $two2 - 1";
?>