添加和连接时，PHP感到困惑

问题描述

我有以下代码:

<?php

    $a = 1;
    $b = 2;

    echo "sum: " .  $a + $b;
    echo "sum: " . ($a + $b);

?>

执行代码时，我得到:

2
sum: 3

为什么在第一个回显中无法打印字符串"sum:"?将加号放在括号中似乎很好.

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

两个运算符加法+运算符和串联.运算符都具有相同的运算符优先级，但是由于它们保持关联性，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));

因此，您的第一行首先进行连接，最后以:

So your first line does the concatenation first and ends up with:

"sum: 1" + 2

(现在，由于这是一个数字上下文，因此您的字符串被转换为整数，因此您最终得到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.)

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