php: About variables, types, scope

Judge whether the variable exists

<code><span>isset</span>(<span>$var</span>);<span>//只判断是否有这个变量，变量存在就是 TURE；</span></code>

Judge whether the variable is empty

<code>！empty(<span>$var</span>);<span>//</span>“”，<span>0</span>，“<span>0</span>”，<span>NULL</span>，<span>FALSE</span>，array(),var <span>$var</span>; 和没有任何属性的对象在用 empty判断时，都是空的，返回<span>TURE</span>；</code>

Judge whether it is false

<code><span>if</span>(<span>0</span> == <span>false</span>){<span>echo</span><span>"true"</span>;}    <span>//true</span><span>if</span>(<span>0.0</span> == <span>false</span>){<span>echo</span><span>"true"</span>;}  <span>//true</span><span>if</span>(<span>array</span>() == <span>false</span>){<span>echo</span><span>"true"</span>;}  <span>//true</span><span>if</span>(<span>"0"</span> == <span>false</span>){<span>echo</span><span>"true"</span>;}  <span>//true</span><span>if</span>(<span>''</span> == <span>false</span>){<span>echo</span><span>"true"</span>;}   <span>//true</span><span>if</span>(<span>NULL</span> == <span>false</span>){<span>echo</span><span>"true"</span>;} <span>//true</span></code>

Static variables: memory-resident variables

<code><span><span>function</span><span>a</span><span>()</span>{</span><span>static</span><span>$a</span> = <span>1</span>;
    <span>echo</span><span>$a</span>;
    <span>$a</span>++;
}
a();<span>//1</span>
a();<span>//2</span></code>

Constant: read-only variables

<code><span><span>define</span></span>(<span>"TEST"</span>,<span>'codekissyoung'</span>);
echo TEST;<span>//codekissyoung</span></code>

Variable variables: PHP is a dynamic real-time analysis The language

<code><span>$a</span> = <span>"test"</span>;
<span>$test</span> = <span>"i am the test"</span>;
<span><span>function</span><span>test</span>（）{</span><span>echo</span><span>"i am function test!"</span>;}   
<span>echo</span><span>$a</span>; <span>//test</span><span>echo</span><span>$$a</span>; <span>// i am the test</span><span>$a</span>(); <span>//i am function test!</span></code>

disassembles the assoc array into variables

<code><span>foreach</span> (<span>$_POST</span><span>as</span><span>$key</span> => <span>$value</span>) {
            <span>$$key</span> = <span>$value</span>;
        ｝</code>

dynamic new objects and call functions

<code><span>//example.com?class=person&func=run</span><span>$class</span>=<span>$_GET</span>[<span>'class'</span>];
<span>$func</span>=<span>$_GET</span>[<span>'func'</span>];
<span>$obj</span>=<span>new</span><span>$class</span>();
<span>$obj</span>-><span>$func</span>();</code>

References to variables

<code><span>$a</span>=<span>"ABC"</span>; 
<span>$b</span> =&<span>$a</span>; 
<span>echo</span><span>$a</span>;<span>//这里输出:ABC </span><span>echo</span><span>$b</span>;<span>//这里输出:ABC </span><span>$b</span>=<span>"EFG"</span>; 
<span>echo</span><span>$a</span>;<span>//这里$a的值变为EFG 所以输出EFG </span><span>echo</span><span>$b</span>;<span>//这里输出EFG </span></code>

Use references and functions in combination

<code><span><span>function</span><span>test</span><span>(&<span>$a</span>)</span> {</span><span>$a</span>=<span>$a</span>+<span>100</span>; 
} 
<span>$b</span>=<span>1</span>; 
test(<span>$b</span>);<span>//这里$b传递给函数的其实是$b的变量内容所处的内存地址，通过在函数里改变$a的值　就可以改变$b的值了 </span><span>echo</span><span>$b</span>;<span>//输出101 </span><span>//要注意的是，在这里test(１);的话就会出错，原因自己去想:标量不能按引用传递</span></code>

Global scope of variables

<code>
(<span>1</span>)现在的程序(web程序，或者其他)都是只有一个入口的(你以 url/test1<span>.php</span> 和 url/test2<span>.php</span> 去访问的其实是两个程序)。

(<span>2</span>)现在的web程序都是只能访问 index<span>.php</span>这个文件，然后index<span>.php</span>去调度资源，返回相应的页面！如果使用了框架的话，index<span>.php</span> 里面会先加载这个框架(资源，类库，函数库)，然后由这个框架来调度资源。

(<span>3</span>)ci就是一个url对应一个类里面的方法，每一次url请求，在服务器端都是执行一个类(控制器)的一个方法而已。而这个方法要如何写，就是我们要实现的程序部分了。换句话说，框架又将调度资源的规则交到了你的手里。

(<span>4</span>)如果想在框架里面使用一些自己定义的函数或者类库，或者变量和常量，可以在index<span>.php</span> 引入框架之前，将你的类库/函数库/常量变量配置文件引入。然后在程序的任意处都可以使用了（除变量）。注意，你的命名不能和php函数以及框架里面的函数重名，否则会被覆盖。

(<span>5</span>)全局变量在函数内部不可以直接访问，要用global 声明一下才可以访问。全局变量在函数外部是可以随意访问的。
</code>

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The above introduces PHP: about variables, types, scopes, including aspects. I hope it will be helpful to friends who are interested in PHP tutorials.