如何修复 PHP 中的“未定义变量”错误？
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How do I fix "Undefined variable" error in PHP?
提问by Anand Mistry
Today, I have started to learn PHP. And, I have created my first PHP file to test different variables. You can see my file as follows.
今天，我开始学习PHP。而且，我已经创建了我的第一个 PHP 文件来测试不同的变量。你可以看到我的文件如下。
<?php
$x = 5; // Global scope
function myTest()
{
$y = 10; // Local scope
echo "<p>Test variables inside the function:<p>";
echo "Variable x is: $x";
echo "<br>";
echo "Variable y is: $y";
}
myTest();
echo "<p>Test variables outside the function:<p>";
echo "Variable x is: $x";
echo "<br>";
echo "Variable y is: $y";
?>
I have found the following errors when I have run this file in the browser.
当我在浏览器中运行这个文件时，我发现了以下错误。
Notice: Undefined variable: x in /opt/lampp/htdocs/anand/php/index.php on line 19
Notice: Undefined variable: y in /opt/lampp/htdocs/anand/php/index.php on line 29
注意：未定义变量：第 19 行 /opt/lampp/htdocs/anand/php/index.php 中的 x
注意：未定义变量：y in /opt/lampp/htdocs/anand/php/index.php on line 29
How can I fix the issue regarding it?
我该如何解决有关它的问题？
回答by meagar
The first error ($xis undefined) is because globals are not imported into functions by default (as opposed to "super globals", which are).
第一个错误（$x未定义）是因为默认情况下全局变量未导入到函数中（与“超级全局变量”相反）。
You need to tell your function you're referencing the global variable $x:
您需要告诉您的函数您正在引用全局变量$x：
function myTest()
{
global $x; // $x refers to the global variable
$y=10; // local scope
echo "<p>Test variables inside the function:<p>";
echo "Variable x is: $x";
echo "<br>";
echo "Variable y is: $y";
}
Otherwise, PHP cannot tell whether you are shadowing the global variable with a local variable of the same name.
否则，PHP 无法判断您是否使用同名的局部变量隐藏全局变量。
The second error ($yis undefined), is because local scope is just that, local. The whole point of it is that $ydoesn't "leak" out of the function. Of course you cannot access $ylater in your code, outside the function in which it is defined. If you could, it would be no different than a global.
第二个错误（$y未定义）是因为本地范围就是本地范围。它的全部意义在于$y不会“泄漏”出函数。当然，您不能$y在代码中稍后访问定义它的函数之外的内容。如果可以，它与全局没有什么不同。
回答by himanshu bhardiya
Set $x as a global, like
将 $x 设置为全局，例如
global $x;
Or try this:
或者试试这个：
<?php
$x = 5; // Global scope
function myTest($x)
{
$y=10; // Local scope
echo "<p>Test variables inside the function:<p>";
echo "Variable x is: $x";
echo "<br>";
echo "Variable y is: $y";
}
myTest($x);
echo "<p>Test variables outside the function:<p>";
echo "Variable x is: $x";
echo "<br>";
echo "Variable y is: $y";
?>
回答by R R
<?php
$a = 1; /* Global scope */
function test()
{
echo $a; /* Reference to local scope variable */
}
test();
?>
You are getting the first error because the variable $a can't access the global variable's value unless you explicitly declare global $ainside the function.
您收到第一个错误是因为变量 $a 无法访问全局变量的值，除非您global $a在函数内显式声明。
Example #1 Using a global
Example #1 使用全局
<?php
$a = 1;
$b = 2;
function Sum()
{
global $a, $b; // If you want to access a global variable,
// you have to use the 'global' keyword
$b = $a + $b;
}
Sum();
echo $b;
?>
And the last error you are getting because $yis defined inside the function mytest()so its scope will be limited to that function only.
你得到的最后一个错误是因为$y是在函数内部定义的，mytest()所以它的范围将仅限于该函数。
For a detailed explanation, read Variable scope.
有关详细说明，请阅读变量范围。
回答by Rajesh Paul
There are two cases of using a variable globally:
全局使用变量有两种情况：
- Using a single copy of that variable and modify it from anywhere i.e. modification from within a function or outside i.e. in the global scope. In that case you need a declaration in the allowed set of function in the form
global $x;. - In case you need local variables for individual functions with the same identifierused for the global variable (i.e. variables outside all functions); in that case you have two variables with the same name i.e. one local and one global for that function. Then you need to use a superglobalvariable
$GLOBALSi.e. an array of all the global variables. I personally prefer this approach to make efficient code;
- 使用该变量的单个副本并从任何地方修改它，即从函数内部或外部即全局范围内修改。在这种情况下，您需要在表单中允许的函数集中进行声明
global $x;。 - 如果您需要具有用于全局变量的相同标识符的单个函数的局部变量（即所有函数之外的变量）；在这种情况下，您有两个同名变量，即该函数的一个局部变量和一个全局变量。然后你需要使用一个超全局变量，
$GLOBALS即所有全局变量的数组。我个人更喜欢这种方法来制作高效的代码；
The following are the code for the two.
以下是两者的代码。
Code 1 (using global declaration)
代码 1（使用全局声明）
<?php
$x = 5; // Global scope
function myTest()
{
$y = 10; // Local scope
global $x;
echo "<p>Test variables inside the function:<p>";
echo "Variable x in global scope is: $x";
echo "<br>";
echo "Variable y is: $y";
}
myTest();
echo "<p>Test variables outside the function:<p>";
echo "Variable x is: $x";
echo "<br>";
echo "Variable y is: $y";
?>
Code 2 (using the $GLOBALS[] array)
代码 2（使用 $GLOBALS[] 数组）
<?php
$x = 5; // Global scope
function myTest()
{
$y = 10; // Local scope
$x = 23;
echo "<p>Test variables inside the function:<p>";
echo "Variable x in global scope is: " . $GLOBALS['x'];
echo "<br>Variable x in local scope is: $x";
echo "<br>";
echo "Variable y is: $y";
}
myTest();
echo "<p>Test variables outside the function:<p>";
echo "Variable x is: $x";
echo "<br>";
echo "Variable y is: $y";
?>
For REFERENCE.
供参考。
回答by Vipin Kumar Soni
The code is behaving as expected, but if you want to use both the variables across the script use this:
代码按预期运行，但如果您想在脚本中同时使用这两个变量，请使用以下命令：
<?php
$x = 5; // Global scope
function myTest(){
global $x;
global $y;
$y = 10;
echo "<p>Test variables inside the function:<p>";
echo "Variable x is: $x";
echo "<br>";
echo "Variable y is: $y";
}
myTest();
echo "<p>Test variables outside the function:<p>";
echo "Variable x is: $x";
echo "<br>";
echo "Variable y is: $y";
?>
回答by Lingasamy Sakthivel
In PHP, global variables must be declared global inside a function if they are going to be used in that function.
在 PHP 中，如果要在该函数中使用全局变量，则必须在该函数内将其声明为 global。
function myTest()
{
$y = 10; // Local scope
global $x;
.....
}
By declaring $x global within the function, it will refer to the global version of the variable.
通过在函数中声明 $x global，它将引用变量的全局版本。
回答by gvgvgvijayan
You have to learn the scope of a variable in PHP. See Variable scope.
您必须了解 PHP 中变量的作用域。请参阅变量范围。
In your code, $x is a global one, so in order to access that variable inside the function use global $x;at the beginning of the function, that is,
在您的代码中， $x 是一个全局变量，因此为了访问函数内部的变量，请在函数global $x;的开头使用，即，
function myTest()
{
global $x;
$y = 10; // Local scope
echo "<p>Test variables inside the function:<p>";
echo "Variable x is: $x";
echo "<br>";
echo "Variable y is: $y";
}
For $y, either you skip that output by checking isset($y) or else assign a default value at the global scope.
对于 $y，要么通过检查 isset($y) 跳过该输出，要么在全局范围内分配默认值。
