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