如何解决$ {}在PHP语法中是什么意思？？

${ }（美元大括号）被称为 ：

之所以称其为“复杂”，是因为语法复杂，而是因为它允许使用复杂的表达式。

可以通过此语法包括具有字符串表示形式的任何标量变量，数组元素或对象属性。只需以与出现在字符串外部相同的方式编写表达式，然后将其包装在{和中即可}。由于{无法转义，因此仅在$紧随其后的才会识别此语法{。使用{\$得到文字{$。一些示例可以使您清楚：

<?PHP
// Show all errors
error_reporting(E_ALL);

$great = ''fantastic'';

// Won''t work, outputs: This is { fantastic}
echo "This is { $great}";

// Works, outputs: This is fantastic
echo "This is {$great}";
echo "This is ${great}";

// Works
echo "This square is {$square->width}00 centimeters broad.";


// Works, quoted keys only work using the curly brace Syntax
echo "This works: {$arr[''key'']}";


// Works
echo "This works: {$arr[4][3]}";

// This is wrong for the same reason as $foo[bar] is wrong  outside a
// string. In other words, it will still work, but only because PHP
// first looks for a constant named foo; an error of level E_NOTICE
// (undefined constant) will be thrown.
echo "This is wrong: {$arr[foo][3]}";

// Works. When using multi-dimensional arrays, always use braces around
// arrays when inside of strings
echo "This works: {$arr[''foo''][3]}";

// Works.
echo "This works: " . $arr[''foo''][3];

echo "This works too: {$obj->values[3]->name}";

echo "This is the value of the var named $name: {${$name}}";

echo "This is the value of the var named by the return value of "
      . " getName(): {${getName()}}";

echo "This is the value of the var named by the return value of "
        . "\$object->getName(): {${$object->getName()}}";

// Won''t work, outputs: This is the return value of getName():

{getName()} echo “This is the return value of getName(): {getName()}”; ?>

解决方法

${  }

$m = new Mongo("mongodb://${username}:${password}@host");

kill `cat -- $PIDFILE`

kill `cat $PIDFILE`

$cat $PIDFILE
cat (GNU coreutils) 6.10
copyright (C) 2008 Free Software Foundation,Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY,to the extent permitted by law.

Written by Torbjorn Granlund and Richard M. Stallman.

$cat -- $PIDFILE
cat: --version: No such file or directory

#define BUFF_SIZE 96-48

1 #include <stdio.h>
2   
3 #define BUFF_SIZE 96-48
4  
5 int main(void) {
6     printf("%d\n",BUFF_SIZE);
7     return 0;
8 }