PHP添加和连接困惑时 [英] Php confused when adding and concatenating

问题描述

考虑这个PHP code：

Consider this php code:

<?php
$row = array('a'=>1,'b'=>2,'c'=>3);
echo "sum: ".$row['a']+$row['b']+$row['c'];
echo "\n";
echo "sum: ".($row['a']+$row['b']+$row['c']);
echo "\n";
?>

在执行形式的终端输出是：

when executed form a terminal the output is:

$ php code.php 
5
sum: 6

为什么在第一回波的总和失败，字符串总和：不打印？似乎PHP是不麻烦时$行中的元素之和括在括号中。 PHP是这种怪异的行为记录？

why in the first echo the sum fails and the string "sum: " is not printed? It seems that php is not in trouble when the sum of the elements of $row is enclosed in parenthesis. Is this weird behaviour of php documented?

谢谢！

更新：看来，这是由于级联和运营商总和的precedence，正如你们许多人指出。非常有趣。感谢你们！！

Update: It seems that this is due to the precedence of the concatenation and sum operators, as many of you pointed. Very interesting. Thank you guys!!

推荐答案

似乎在PHP中的 + 运营商具有较高的precedence比。（即。结合在前）。你的第一个前pression然后进行评估，如：

Seems that in PHP the + operator has higher precedence than the . (i.e. . binds first). Your first expression then is evaluated like:

("sum: ".$row['a'])+$row['b']+$row['c']
("sum: 1"+$row['b'])+$row['c']
2+$row['c']
5

（添加字符串总和：1来一个数字，字符串是<一个href=\"http://www.php.net/manual/en/language.types.string.php#language.types.string.conversion\">coerced以零值，使之和的结果是 2 ）

(when adding the string "sum: 1" to a number, the string is coerced to the value zero, so the result of the sum is 2)

更新：其实他们有相同precedence ，但因为他们是左结合，在。首先计算。

Update: Actually they have the same precedence, but since they're left associative, the . is evaluated first.

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