为什么在eval语句中变量之前存在反斜杠？

Question

While learning PHP online I am stopped at this eval function please help me. Why is there is a slash before $str2 in eval statement?

<?php
$string = "beautiful";
$time = "winter";

$str = 'This is a $string $time morning!';
echo $str. "<br>";

eval("\$str2 = \"$str\";");
echo $str2;
?>

Answer 1

The slash escapes the dollar sign, else in double quotes the dollar signs starts a variable name.

echo $var; // print the content of $var
echo "$var"; // print the content of $var
echo "\$var"; // print '$var'
echo '$var'; // print '$var'

Other thing is that you should find another book/tutorial for studying. Usign eval is unrecommended and in this case bad.

The last two lines of your code should be:

$str2 = $str;
echo $str2;

OR just

echo $str;