为什么，会报错
$love = "I love you!";
$string1="慕课网,$love";
$string2='慕课网,$love';
$string3="conb,$like";
echo $string1;
echo "
";
echo $string2;
echo "
";
echo $string3;
?>
结果
Notice: Undefined variable: like in /54/750/IGfP/index.php on line 6
慕课网,I love you!
慕课网,$love
conb,
怎么会报错
