Detailed explanation of PHP variable scope

The scope of a variable is the context in which it is defined (that is, its effective scope). In JavaScript, there is no concept of variable scope. A similar concept may be scope. However, since JavaScript uses lexical scope, which refers to the position when the variable is declared; and PHP does not have variable declaration, the variable is equivalent to declaring the variable when it is assigned a value for the first time. Therefore, the two are not the same. This article will introduce variable scope in php in detail.

1. Local variables
Local variables are variables declared within a function, and their scope is limited to the inside of the function. The parameters of a function are also local variables. The difference is that the function parameter value comes from the value passed in when it is called. After execution, all dynamic variables inside the function will be released.

<?php
    function a($b){
        $c = 1;        echo "$b+$c = " . ($b+$c) . &#39;<br />&#39;;
    }
    a(2);//输出3
    echo $b+$c;//函数外部的非法访问，报notice错误，并输出0
?>

2. Global variables
Variables defined outside the function are called global variables, and their scope ranges from the beginning of the defined variable to the end of the program file.
In addition, global variables cannot be called directly in the function. You need to use the keyword global. You can also use $GLOBALS instead of global. $GLOBALS is a superglobal variable.

<?php
    //示例1
    $a = 1;    $b = 2;    function Sum(){
        global $a, $b;        $b = $a + $b;
    }
    Sum();    echo $b;//输出的结果是3

    //示例2
    $c = 1;    $d = 2;    function Sum()
    {
        $GLOBALS[&#39;d&#39;] = $GLOBALS[&#39;c&#39;] + $GLOBALS[&#39;d&#39;];
    }
    Sum();    echo $d;//输出3
?>

3. Static variables
Static variables only exist in the local function scope, but when the program execution leaves this scope, its value is not lost. Static variables are declared with static, and those that are not statically declared are dynamic variables by default. After the function is executed, the internal static variables are already stored in memory. It is only initialized the first time the function is called during script execution.

<?php
    function test()
    {
        static $a = 0;        echo $a;        $a++;
    }
    test();//输出0
    test();//输出1
?>

Static variables also provide a way to deal with recursive functions. A recursive function is a function that calls itself. Be careful when writing recursive functions, as they may recurse indefinitely. You must ensure that there are adequate ways to terminate recursion. The following simple function recursively counts to 10, using the static variable $count to determine when to stop

<?php
    function test()
    {
        static $count = 0;        $count++;        echo $count;        if ($count < 10) {
            test();
        }        $count--;
    }
?>

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