如何调试file_get_contents（）中的HTTP提取无法正常工作的原因

I am trying to use file_get_contents in different way. Somehow it's not working by this way. Please see the code

<?php
$attrb = "https://html5andcss3.org";
$htmlcontent = file_get_contents("'" .$attrb . "'");
echo $htmlcontent;
?>

I got the error message

But when I use this code in normal way, it works fine. Please see the working code below

<?php
$htmlcontent = file_get_contents('https://html5andcss3.org');
echo $htmlcontent;
?>

My problem is that, I will get the URL in a variable and I need to start from there. So I can not directly put the URL.