Can a script be written in PHP to receive standard e-mails, and what tools would be required to do so, including libraries and functions? Furthermore, is there a pre-existing script with a registered e-mail accessible via a URL that can be logged into by forwarding the login and password? If this is possible, instructions on how to achieve it would be appreciated. If there are any available scripts or resources on this topic, any information would be greatly appreciated.

A multitude of PHP webmail programs can be found online, such as NOCC, a webmail client that uses IMAP and POP3 and can be hosted at home. Alternatively, there are existing services that allow you to access your POP3 mail remotely through the internet, including Mail2Web.
Regarding the question of entering a page generated by another script, it's unclear what is being asked. If the purpose is to display the content of a web page to users, CURL can be used to achieve this. The provided code snippet shows an example of how to use CURL to fetch a web page with login credentials and store its contents in a variable.

Expanding on this topic, CURL is a powerful tool that can be used for various tasks related to web development. It enables PHP scripts to communicate with remote servers via HTTP, HTTPS, FTP, and more, allowing for the retrieval and manipulation of web pages, data, and files. However, like with any web automation tool, it's important to use CURL ethically and follow best practices to avoid violating web scraping policies or using malicious intents.

<?php
$url="http://mail4web.com";
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt ($ch, CURLOPT_POST, 1);
curl_setopt ($ch, CURLOPT_POSTFIELDS, "mailaddress=youremail&password=yourpassword&");
curl_setopt ($ch, CURLOPT_RETURNTRANSFER, 1);
$store = curl_exec ($ch);
$content = curl_exec ($ch);
curl_close ($ch); 
?>

While there are software packages that can achieve this, basic tools like echo, sleep, and netcat can be used to complete the same tasks. The process involves connecting to a server via POP3 and using a specified username and password to retrieve and delete one email before disconnecting. However, additional protocol messages are also generated alongside the email, which can be ignored or trimmed down.

A custom bash function is provided in the text that facilitates this process. To use it, one must call it on the desired server with their username and password. If required, the text encoding can be adjusted to suit your preferences, and the fixed delay (sleep 5) in receiving emails can be adjusted accordingly.

Some servers, notably Google, do not allow simple POP3 connections and only support encrypted connections. To work around this, the stunnel program can be used to create an encrypted tunnel between the client and server, providing a secure connection to Gmail's POP3 and SMTP services.

While accessing webmail through the shell can be a valuable tool for automating certain tasks, it requires a certain level of technical knowledge and may not be suitable for all users. Additionally, certain email providers have policies against automated access to their services, so it's crucial to review the terms and conditions before attempting to use these methods.

You will need the PHP IMAP extension, which provides functions to manipulate and search through emails in a server mailbox, and the PHP mail function to send emails.

First, make sure that the IMAP extension is enabled in your PHP configuration. You can check this by creating a PHP file with the `phpinfo()` function and searching for 'IMAP' in the output.

To receive emails in PHP, you can use the IMAP functions to connect to a mail server, authenticate using a username and password, and then retrieve emails from the inbox. Here's a basic example to get you started:

$hostname = '{mail.example.com:993/imap/ssl}INBOX';
$username = 'your_email@example.com';
$password = 'your_password';

$inbox = imap_open($hostname, $username, $password) or die('Cannot connect to mailbox: ' . imap_last_error());

$emails = imap_search($inbox, 'UNSEEN');

if ($emails) {
    foreach ($emails as $email_number) {
        $email_header = imap_headerinfo($inbox, $email_number);
        $subject = $email_header->subject;
        $from = $email_header->fromaddress;
        $body = imap_fetchbody($inbox, $email_number, 1);
        // Process the email further, for example, store it in a database or perform some action based on its content
    }
}

imap_close($inbox);

For sending emails using PHP, you can use the `mail()` function. Here's a simple example:

$to = 'recipient@example.com';
$subject = 'Hello from PHP!';
$message = 'This is a test email';
$headers = 'From: your_email@example.com';

mail($to, $subject, $message, $headers);

Regarding the pre-existing script with a registered email accessible via a URL that can be logged into by forwarding the login and password, it's important to note that accessing email accounts via URL using login and password forwarding is generally not recommended due to security risks such as exposing login credentials in the URL. Instead, consider using secure authentication methods like OAuth or API keys for accessing email accounts programmatically.

Finally, always remember to handle sensitive information such as email credentials securely, for example by using environment variables or configuration files outside of your script's root directory.

I'd recommend using PHPMailer, a feature-rich and robust email library. It doesn't receive emails, but it's great for sending them. For receiving, you'd still need an IMAP server or a service like Gmail's IMAP.

require 'PHPMailer/src/Exception.php';
require 'PHPMailer/src/PHPMailer.php';
require 'PHPMailer/src/SMTP.php';

$mail = new PHPMailer\PHPMailer\PHPMailer();
$mail->isSMTP();
$mail->Host = 'smtp.example.com';
$mail->SMTPAuth = true;
$mail->Username = 'username';
$mail->Password = 'password';

if ($mail->send()) {
    echo 'Email sent!';
} else {
    echo 'Email could not be sent.';
}