单引号与双引号的区别

如果是单引号，PHP不解析其中的内容，直接当作一个字符串操作，如果是双引号，PHP会自动用变量的值替换掉变量名称，所以在处理字符串的时候最好使用单引号，这样速度快一些
如：  $a = 5;
      echo "$a";    //5
      echo '$a';    //$a

双引号字符串最重要的一点是其中的变量名会被变量值替代
$var1 = "Chris Mao";
$var2 = &$var1;    //引用传递$var1的值随着$var2的变化而变化
echo 'The value of $var2 is: ', $var2, "
";  
//值是The value of $var2 is: Chris Mao

$var2 = 'my name is $var2';  
echo 'The value of $var1 is: ', $var1, "
";  
//值是The value of $var1 is: my name is $var2

$var2 = "my New name is $var1";
//在这个双引号里面就已经把$var1变量前面的值给替代了
echo 'The value of $var2 is: ', $var2, "
";
//值是The value of $var2 is: my New name is my name
  is $var2

echo 'The value of $var1 is: ', $var1, "
";
//值是The value of $var2 is: my New name is my name
  is $var2