PHP Fatal error: Cannot redeclare solution

In the process of using PHP for development, sometimes you will encounter the "PHP Fatal error: Cannot redeclare" error. This error usually occurs in the following situations:

Include/ multiple times in the PHP code require the same file.
A function/class with the same name as an existing function/class is defined in the code.

This error will cause the program to be unable to continue execution. In order to solve this problem, we need to understand its cause and solution.

Cause

There are generally two reasons for PHP "Cannot redeclare" errors:

1. The file is repeatedly referenced

When using include /require function, if the same file is referenced multiple times, it will cause a "Cannot redeclare" error in PHP.

For example, if we have a file named "test.php":

function test(){

echo "Test function.";

}
?>

Then, when this file is repeatedly referenced in other files, this error will occur:

// File "index.php"
require_once "test .php";
require_once "test.php"; // The same file is referenced here

2. Functions or classes with the same name are defined

PHP will save them in memory Defined functions and classes. If a function/class with the same name as an existing one is defined in the program, a "Cannot redeclare" error will occur.

For example, a function named "test" is defined in the code:

function test() {

echo "Test function.";

}

However, In other code blocks, if a function with the same name is defined, this error will occur:

function test() {

echo "Test function is redefined.";

}

Solution

1. Avoid repeatedly referencing files

In PHP, in order to avoid repeatedly referencing files, we should use the require_once or include_once function.

The require_once function and the include_once function work similarly to the require and include functions, except that they ensure that a file is only referenced once.

We modify the above example:

// File "index.php"
require_once "test.php";
require_once "test.php"; // Already referenced No more references here

2. Avoid defining functions or classes with the same name

In order to avoid defining functions or classes with the same name, we need to pay attention to the code naming convention and code when writing code structure. We can also avoid defining functions or classes with the same name through the following methods:

Method 1: Before defining a function, you can first determine whether the function already exists, for example:

if (!function_exists(' test')) {

function test() {
    echo "Test function.";
}

}

Method 2: Use namespace. PHP 5.3 introduced namespaces. We can use namespaces to distinguish and avoid functions and classes with the same name.

For example:

// Define the Test namespace
namespace Test;

class Test {

public static function sayHello() {
    echo "Hello world!";
}

}

/ / Call the Test class under the Test namespace
use TestTest;

Test::sayHello();

Summary

"PHP Fatal error: Cannot redeclare" error It is one of the common errors in PHP development. The reason is usually that files are repeatedly referenced or function/class names are repeatedly defined. We can avoid this error by using require_once/include_once functions, following naming conventions when writing code, utilizing namespaces, etc. When writing PHP code, we should pay attention to the structure and naming convention of the code to avoid unnecessary trouble caused by similar errors.

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