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一步一步学习PHP(4) php 函数 补充2_php基础

May 17, 2016 am 09:02 AM
php function

1. 解决作用域问题

在上一节谈到了PHP中函数的作用域问题,如果说类还不够说明问题,那么在这篇文章中的作用域例子,也许对您更有说服力。

那么,我们如何能在PHP的函数中访问外部的变量呢?

在PHP中,存在一个概念叫做全局作用域,也就是说,如果你不使用函数(未来会有类的概念),那么你创建的变量可以在页面的任何地方访问。那么,我们怎么解决函数体内的问题呢?

我们可以使用global关键字:

  <SPAN style="COLOR: blue"><?</SPAN><SPAN style="COLOR: maroon">php 
    </SPAN>$<SPAN style="COLOR: red">name</SPAN><SPAN style="COLOR: blue">="kym"</SPAN>;
    <SPAN style="COLOR: red">function Test</SPAN>()
    {
      <SPAN style="COLOR: red">global </SPAN>$<SPAN style="COLOR: red">name</SPAN>;
      <SPAN style="COLOR: red">echo</SPAN>($<SPAN style="COLOR: red">name</SPAN>);
    }
    <SPAN style="COLOR: red">Test</SPAN>();
  <SPAN style="COLOR: blue">?>
</SPAN>
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这样的话,我们就相当于得到了对全局变量$name的一个引用,而不需要重新创建一个$name变量了。

在PHP手册中有这样一句话:使用全局变量等效于在$GLOBALS变量中创建一个对变量的引用。那么我们就可以模仿这个语义写出这样的代码:

  <SPAN style="COLOR: blue"><?</SPAN><SPAN style="COLOR: maroon">php 
    </SPAN>$<SPAN style="COLOR: red">name</SPAN><SPAN style="COLOR: blue">="kym"</SPAN>;
    <SPAN style="COLOR: red">function Test</SPAN>()
    {
      $<SPAN style="COLOR: red">temp</SPAN><SPAN style="COLOR: blue">=&$GLOBALS["</SPAN><SPAN style="COLOR: red">name</SPAN><SPAN style="COLOR: blue">"];
      echo($temp);
    }
    Test();
  ?>
</SPAN>
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其实这两种是等效的代码。看上去这样使用变量很麻烦,但是这样其实避免了很多副作用的产生,如Java,C#是面向对象的语言,但是在PHP中,一切都在一个页面,甚至多个页面中,那么如果不使用这样的办法,很容易产生函数的副作用(误修改)。

2. 默认参数

第一次听过这个概念应该是在C++中,之后学了C#,就没有接触过这个概念了,不过个人还是非常喜欢这个特性的。

这样就可以不必要为一个函数写一堆麻烦的重载函数了。

写个最简单的例子:

  <SPAN style="COLOR: blue"><?</SPAN><SPAN style="COLOR: maroon">php 
    </SPAN><SPAN style="COLOR: red">function Test</SPAN>($<SPAN style="COLOR: red">name</SPAN><SPAN style="COLOR: blue">="kym"</SPAN>)
    {
      <SPAN style="COLOR: red">echo</SPAN>($<SPAN style="COLOR: red">name</SPAN>);
    }
    <SPAN style="COLOR: red">Test</SPAN>();
    <SPAN style="COLOR: red">Test</SPAN>(<SPAN style="COLOR: blue">"others"</SPAN>);
  <SPAN style="COLOR: blue">?>
</SPAN>
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是不是不需要写一个重载函数了呢?

但是在这里需要注意两点:

A. 在为参数设置默认值时,只能设置最简单的常量,而不能包含复杂的表达式。

B. 需要设置默认值的参数必须放在最后。这个类似于C#中的可变参数。

3. 可变参数

在C#中,就存在这样一个概念叫可变参数,写个简单的例子:

<SPAN style="COLOR: blue">protected void </SPAN>LinkButton1_Click(<SPAN style="COLOR: blue">object </SPAN>sender, <SPAN style="COLOR: #2b91af">EventArgs </SPAN>e)
{
  Response.Write(GetSum(1, 2, 3, 4, 5));
}
<SPAN style="COLOR: blue">public int </SPAN>GetSum(<SPAN style="COLOR: blue">params int</SPAN>[] elements)
{
  <SPAN style="COLOR: blue">int </SPAN>sum = 0;
  <SPAN style="COLOR: blue">for </SPAN>(<SPAN style="COLOR: blue">int </SPAN>i = 0; i < elements.Length; i++)
  {
    sum += elements[i];
  }
  <SPAN style="COLOR: blue">return </SPAN>sum;
}
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在写例子之前,我先来介绍三个与可变参数相关的函数。

func_get_args()返回函数的所有参数的数组

func_get_arg()返回一个来自参数的特定参数

func_num_args()返回参数的个数。

好,那么接下来就写一个和C#版本对应的PHP版本。

  <SPAN style="COLOR: blue"><?</SPAN><SPAN style="COLOR: maroon">php 
    </SPAN><SPAN style="COLOR: red">function GetSum</SPAN>()
    {
      <SPAN style="COLOR: red">if</SPAN>(<SPAN style="COLOR: red">func_num_args</SPAN>()<SPAN style="COLOR: blue">=</SPAN>=0)
      {
        <SPAN style="COLOR: red">return </SPAN>0;
      }
      $<SPAN style="COLOR: red">sum</SPAN><SPAN style="COLOR: blue">=0;
      </SPAN><SPAN style="COLOR: red">for</SPAN>($<SPAN style="COLOR: red">i</SPAN><SPAN style="COLOR: blue">=0;$i<</SPAN><SPAN style="COLOR: maroon">func_num_args</SPAN>();$<SPAN style="COLOR: red">i</SPAN>++)
      {
        $<SPAN style="COLOR: red">sum</SPAN>+<SPAN style="COLOR: blue">=func_get_arg($i);
      </SPAN>}
      <SPAN style="COLOR: red">return </SPAN>$<SPAN style="COLOR: red">sum</SPAN>;  
    }
    <SPAN style="COLOR: red">echo</SPAN>(<SPAN style="COLOR: red">GetSum</SPAN>(1,2,3,4));
  ?<SPAN style="COLOR: blue">>
</SPAN>
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4. 可变函数

我不知道为什么要翻译成这个名字,其实就是根据变量的名字来调用相应的函数。

很类似于JS中的这个eval,如下所示:

<SPAN style="COLOR: teal"><</SPAN><SPAN style="COLOR: #3f7f7f">script</SPAN><SPAN style="COLOR: teal">>
  </SPAN><SPAN style="COLOR: #7f0055">function </SPAN>Execute(functionName)
  {
    eval(functionName+<SPAN style="COLOR: #2a00ff">"()"</SPAN>);
  }
  <SPAN style="COLOR: #7f0055">function </SPAN>Test()
  {
    alert(<SPAN style="COLOR: #2a00ff">"111"</SPAN>);
  }
  Execute(<SPAN style="COLOR: #2a00ff">"Test"</SPAN>);
<SPAN style="COLOR: teal"></</SPAN><SPAN style="COLOR: #3f7f7f">script</SPAN><SPAN style="COLOR: teal">></SPAN>
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这个就是传进来一个函数名,然后来执行对应的函数。那么在PHP中,其实也是一样的,而且更简单了。

<SPAN style="COLOR: red"><?php 
  </SPAN><SPAN style="COLOR: blue">function </SPAN>Test(<SPAN style="COLOR: #660000">$func</SPAN>)
  {
    <SPAN style="COLOR: #660000">$func</SPAN>();
  }
  <SPAN style="COLOR: blue">function </SPAN>First(){<SPAN style="COLOR: blue">echo</SPAN>(<SPAN style="COLOR: #008200">"first"</SPAN>);}
  Test(<SPAN style="COLOR: #008200">"First"</SPAN>);
<SPAN style="COLOR: red">?></SPAN>
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