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php编程中echo用逗号和用点号连接的区别,echo逗号
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php编程中echo用逗号和用点号连接的区别,echo逗号

Jun 13, 2016 am 08:43 AM
echo php comma

php编程中echo用逗号和用点号连接的区别,echo逗号

里面提到了echo 字符串用,比用.连接要好。原因先不说,先来看以下两句

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<&#63;php

// 逗号比.更节省时间&#63;

echo '1+5=' . 1+5;

echo '1+5=' . 5+1;

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结果是什么?

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1+5=6?

1+5=6?

——————

6?

2?

——————

6.6?

6.6?

——————

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我只能说echo '5+1=' . 1+5;的结果是10,所以结果是6和2.

为什么会这样.难道在PHP中加法是没有交换律的?当然不是..
我们先不去想为什么.如果我把上面的点号换成逗号试下.

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echo '1+5=' , 5+1;  //输出 1+5=6

echo '1+5=' , 1+5;  //输出 1+5=6

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可以看出.只有使用逗号我们才可以得到意料中的结果.

那为什么点号就不行呢?逗号为什么就行呢?

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echo ('1+5' . 5)+1; //输出2

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我们给前面的加个括号后.得到的结果是一样的.

证明PHP是先连接字符串再进行加法计算了.按照从左向右的方向进行的.

那么好.既然是先连接的字符串.那么就应该是"1+55"了.然后再用这个字符串加上1.那为什么就会输出2呢?

这个跟PHP中字符串变成数字的机制是相关的.我们来看下面的例子

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echo (int)'abc1'//输出0

echo (int)'1abc'; //输出1

echo (int)'2abc'; //输出2

echo (int)'22abc'; //输出22

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从上面的例子我们可以看出.如果将一个字符串强制转换成一个数字.PHP会去搜索这个字符串的开头.如果开头是数字就转换.

如果不是就直接返回0.

回到刚才的1+55.既然这个字符串是1+55.所以强制类型转换后就应该是1了.在此基础上加1.当然是2了.
为了证明我们的猜想.我们来验证一下.

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echo '5+1=' . 1+5; //输出10

echo '5+1=' . 5+1; //输出6

echo '1+5=' . 1+5; //输出6

echo '1+5=' . 5+1; //输出2

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结果证明.我们的设想是正确的.
那么为什么使用逗号就没有上面的问题了呢?

手册上说了.用逗号是multiple parameters .

也就是说是多参数.换句话说.

逗号分隔开的就相当于是N个参数.也就是说把echo当个函数用.

这样的话.echo会对每个参数先进行计算.最后再进行连接后输出.所以我们用逗号就不存在上面的问题了:)

php echo manual

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<&#63;php

// Strings can either be passed individually as multiple arguments or

// concatenated together and passed as a single argument

echo 'This ', 'string ', 'was ', 'made ', 'with multiple parameters.', chr(10);

echo 'This ' . 'string ' . 'was ' . 'made ' . 'with concatenation.' . "\n";

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至于为啥快,可以简单理解,用.是先拼接在echo,虽然逗号个数代表echo的调用次数(可以这么理解暂时)。
但是拼接的速度要小于echo的速度。
如果深入理解,VLD如下图。是@tywei大神的图


上面比下面多了个CONCAT,下面比上面多了个echo。

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