In PHP, what is the difference between declaring a variable as a global internal function or passing a variable as an argument to a function?

Question

What is the difference between declaring a variable inside a function like globalor public/privateVS, passing it to the function as an argument?

Another related confusion

I recently caused a big headache, trying to pass the aa array variable to a function as globalwell as edit it inside and hope to return it, and it took me several hours to realize that I needed to pass it to the function as an argument by reference, for examplefunctionCall(&$arrayVar);

Secondary question: But I'm still wondering why it doesn’t work to pass the variable as a quality global, then edit it and return it with returnor just by doing something like concatenation into an array of variables?

Another example that I recently encountered is creating a function for PHPMailer, where I pass it a few arguments, such as an email address and message body, but I also need to pass authentication strings, such as an API key, etc. Here, every time I call him:

• I don’t want to give him authentication credentials every time I call the PHMailer function (for example, send an email error message in one of several stages)
• But I want to give him unique arguments every time I call him

, , :

function phpMailer( $mail_to = "[e-mail]", $mail_from = "[e-mail]" ) {
    global $authCredentials;
}

// And of course, when I call phpMailer, I call it like
phpMailer("[e-mail]", "[e-mail]");

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Answer 1

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define('AUTH', 'my_key');
function phpMailer( $mail_to = "[e-mail]", $mail_from = "[e-mail]" ) {
    echo AUTH;
}

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class phpMailer()
{
    private $auth = 'my_key';

    public function send($mail_to, $mail_from)
    {
        $this->auth;
    }
}
$mail = new phpMailer();
$mail->send('[e-mail]', '[e-mail]');

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