<?php
$string1 = 'string1';
$string2 = "string2";
$string3 = 'string3$string1';//$string3变量的值为：string3$string1
$string4 = "string4$string1";//$string4变量的值为：string4string1
echo $string1;
echo "<br/>";
echo $string2;
echo "<br/>";
echo $string3;
echo "<br/>";
echo $string4;
?>