var_dump(1...9)输出什么?

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一个问题, var_dump(1...9)输出什么?

动手验证下:

php -r “var_dump(1...9)”;
string(4) ”10.9“

输出10.9, 乍一看这个var_dump的输出很奇怪是不是？为什么呢？

这里教大家，如果看到一段PHP代码感觉输出很奇怪，第一反应是看下这段代码生成的opcodes是啥，虽然这个问题其实是词法分析阶段的问题，不过还是用phpdbg分析下吧(一般为了防止opcache的影响，会传递-n):

phpdbg -n -p /tmp/1.php
function name: (null)
L1-35 {main}() /tmp/1.php - 0x7f56d1a63460 + 4 ops
L2 #0 INIT_FCALL<1> 96 "var_dump"
L2 #1 SEND_VAL "10.9" 1
L2 #2 DO_ICALL
L35 #3 RETURN<-1> 1

所以这么看来，早在生成opcode之前，1...9就变成了常量10.9，考虑到这是字面量，我们现在去看看zend_language_scanner.l, 找到这么一行:

DNUM ({LNUM}?"."{LNUM})|({LNUM}"."{LNUM}?)

这个是词法分析定义的浮点数的格式，到这里也就恍然大悟了：
1...9 会被依次接受为: 1. (浮点数1), 然后是 . (字符串连接符号）然后是.9(浮点数0.9)

所以在编译阶段就会直接生成 “1” . “0.9” -> 字符串的字面量”10.9”

好了，到这里，这个小“谜题”就解释清楚了。

当然这个也不仅仅是PHP会这么定义了，几乎所有的语言都会定义这种缩写浮点数都形式. C语言中有的时候我们为了输入一个浮点型的整形，就可以采用比如 1. 来告诉编译器这是个浮点数.

只不过，一来刚好在PHP中.号还有另外一层含义就是字符串连接，二来...在PHP5.6之后是个新的操作符叫做Splat operator, 可以用来定义可变参数函数，或者解数组，比如,

<?php

function  foo($a, $b, $c) {
        var_dump($a + $b + $c);
}


$parameters = array (1, 2, 3);

foo(...$parameters);
?>

所以，乍一看才会导致了这个看起来很困惑的结果, 🙂

16 Comments

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maple June 29, 2020

无力吐槽（
phper April 7, 2020

这样写代码会被看你代码的人骂的，所以还是不要这样写…… ^_^
阿毛 March 27, 2020

这样写代码会被打死系列
var_dump(1…9)输出什么? – 便民资讯网 February 25, 2020

[…] by lovekay·0评论 var_dump(0…9)输出什么? php […]
沈唁志 February 25, 2020

第一个代码块中的引号是中文的🧐
- laruence February 25, 2020
  
  wordpress特别2 ，自动转
石樱灯笼 February 25, 2020

新的PHP面试题（坑）诞生了
- jaggle July 27, 2020
  
  这种当面试题，你还是不要坑别人了。不是神仙都做不出来

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var_dump(1...9)输出什么?

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