问题描述
我有以下代码:
<?php
$a = 1;
$b = 2;
echo "sum: " . $a + $b;
echo "sum: " . ($a + $b);
?>
当我执行我的代码时，我得到:
When I execute my code I get:
2
sum: 3
为什么在第一个回显中打印字符串"sum:" 失败?加法用括号括起来似乎没问题.
Why does it fail to print the string "sum:" in the first echo? It seems to be fine when the addition is enclosed in parentheses.
这种奇怪的行为在任何地方都有记录吗?
Is this weird behaviour anywhere documented?
推荐答案
加法 + 运算符和连接 . 运算符都有相同的 运算符优先级，但由于它们是关联的，因此它们的评估如下:
Both operators the addition + operator and the concatenation . operator have the same operator precedence, but since they are left associative they get evaluated like the following:
echo (("sum:" . $a) + $b);
echo ("sum:" . ($a + $b));
所以你的第一行首先进行连接，最后是:
So your first line does the concatenation first and ends up with:
"sum: 1" + 2
(现在因为这是一个数字上下文，你的 字符串被转换为整数，因此你最终得到0 + 2，然后得到结果2.)
(Now since this is a numeric context your string gets converted to an integer and thus you end up with 0 + 2, which then gives you the result 2.)
这篇关于PHP在添加和连接时感到困惑的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持编程学习网！