为什么我得到“无法重新声明课程”错误？

I have Apache running on port 81. My project folder is MyPhpProject. Inside it I have 2 folders: Domain and Testing.

In Domain folder I have 3 PHP files:

1. BaseDomain.php which contains an abstract class BaseDomain
   
2. Location.php which contains a concrete class Location inherited from BaseDomain
   
3. Employee.php which contains a concrete class Employee inherited from BaseDomain
   

Employee class has a reference of Location class.

This is the BaseDomain.php:

<?php
abstract class BaseDomain {

}
?>

This is the Location.php:

<?php
$returnRequire = require 'BaseDomain.php';

class Location extends BaseDomain {

    private $locationIdInt;         
    private $codeNameString;        
    private $descString;        

    public function setLocationId($locationId) {
        $this->locationIdInt = $locationId;
    }
    public function getLocationId() {
        return $this->locationIdInt;
    }

    public function setCodeName($codeName) {
        $this->codeNameString = $codeName;
    }
    public function getCodeName() {
        return $this->codeNameString;
    }

    public function setDesc($desc) {
        $this->descString = $desc;
    }
    public function getDesc() {
        return $this->descString;
    }
}
?>

This is Employee.php:

<?php
$returnRequire = require 'BaseDomain.php';

class Employee extends BaseDomain {

    private $employeeIdString;          
    private $locationObject;            

    public function setEmployeeId($employeeId) {
        $this->employeeIdString = $employeeId;
    }
    public function getEmployeeId() {
        return $this->employeeIdString;
    }

    public function setLocation($location) {
        $this->locationObject = $location;
    }
    public function getLocation() {
        return $this->locationObject;
    }
}
?>

Now in the Testing folder I created a Test_Employee.php and this is its code:

<?php
set_include_path('../Domain');

$getIncludePath = get_include_path();
echo "getIncludePath = " . $getIncludePath;
echo "<br>";

$returnRequire1 = require 'Location.php';
echo "returnRequire for Location.php = " . $returnRequire1;
echo "<br>";

$returnRequire2 = require 'Employee.php';
echo "returnRequire for Employee.php = " . $returnRequire2;
echo "<br>";
?>

When I try to run it http://localhost:81/MyPhpProject/Testing/Test_Employee.php I got a fatal error regarding cannot redeclare BaseDomain class. This is what I see in browser:

getIncludePath = ../Domain

returnRequire for Location.php = 1

Fatal error: Cannot redeclare class BaseDomain in C:\Program Files (x86)\Apache Software Foundation\Apache2.2\htdocs\MyPhpProject\Domain\BaseDomain.php on line 2

I have not created BaseDomain class more than once. So this error is bizarre. Can somebody please explain why I am getting error message? And how to fix it.

Thanks for your time.

The line $returnRequire1 = require 'Location.php'; loads Location.php, which in turns loads BaseDomain.php in the line $returnRequire = require 'BaseDomain.php';. Then, the line $returnRequire2 = require 'Employee.php'; loads Employee.php, which loads (again) BaseDomain.php (the line $returnRequire = require 'BaseDomain.php';). The second load of BaseDomain.php causes php to try to redefine the BaseDomain class, which is no allowed.

The easiest way to solve this problem is to change your require calls to require_once. This will ensure that each file is loaded exactly once per run, which will prevent the error you are experiencing.

BaseDomain.php:

<?php
abstract class BaseDomain {

}
?>

Location.php

<?php
class Location extends BaseDomain {

    private $locationIdInt;         
    private $codeNameString;        
    private $descString;        

    public function setLocationId($locationId) {
        $this->locationIdInt = $locationId;
    }
    public function getLocationId() {
        return $this->locationIdInt;
    }

    public function setCodeName($codeName) {
        $this->codeNameString = $codeName;
    }
    public function getCodeName() {
        return $this->codeNameString;
    }

    public function setDesc($desc) {
        $this->descString = $desc;
    }
    public function getDesc() {
        return $this->descString;
    }
}
?>

Employee.php:

<?php

class Employee extends BaseDomain {

    private $employeeIdString;          
    private $locationObject;            

    public function setEmployeeId($employeeId) {
        $this->employeeIdString = $employeeId;
    }
    public function getEmployeeId() {
        return $this->employeeIdString;
    }

    public function setLocation($location) {
        $this->locationObject = $location;
    }
    public function getLocation() {
        return $this->locationObject;
    }
}
?>

Test_Employee.php

<?php
set_include_path(__DIR__.'/MyPhpProject/Domain');
require 'BaseDomain.php';

$getIncludePath = get_include_path();
echo "getIncludePath = " . $getIncludePath;
echo "<br>";

$returnRequire1 = require 'Location.php';
echo "returnRequire for Location.php = " . $returnRequire1;
echo "<br>";

$returnRequire2 = require 'Employee.php';
echo "returnRequire for Employee.php = " . $returnRequire2;
echo "<br>";
?>