include_once或require_once解决了哪些问题？

Question

From the php manual

Include_once may help avoid problems such as function redefinitions, variable value reassignments, etc.

Ok, so include_once solves issues with function redefinitions, variable value reassignments, etc. but why are they an issue in the first place ?

I'm trying to understand what kind of risks are involved in redefining functions or reassigning variable values except for a decline in performance due to additional input/output and processing ?

Is it because php parser gets confused which version of function to load/use or is the original version of the function lost once redefined? What else and what about variable reassignments?

I do understand where to use include vs include_once.

Answer 1

Imagine the following include file, hello.php:

function hello()
{
    return 'Hello World';
}

$a = 0;

Now imagine the following file, index.php:

include 'hello.php';

$a = 1;
hello();

include 'hello.php';

hello();
echo $a; // $a = 0, not 1

Your code would now have a fatal error, since the function has been defined twice. Using include_once would avert this, since it would only include hello.php once. Also, to do with variable value reassignment, $a (should the code compile) would be reset to 0.

---

From the comments, please consider this a side answer - If you're looking for something where resetting a set of variables many times was required, I'd look to use a class for this with a method like Reset, you can even make it static if you didn't want to have to instantiate it, like so:

public class MyVariables
{
    public static $MyVariable = "Hello";
    public static $AnotherVariable = 5;

    public static function Reset()
    {
        self::$MyVariable = "Hello";
        self::$AnotherVariable = 5;
    }
}

Usage like:

MyVariables::$MyVariable = "Goodbye";
MyVariables::Reset();
echo MyVariables::$MyVariable; // Hello