How can PHP be used to create a file selection dialog for users to attach files to an email?

To create a file selection dialog for users to attach files to an email using PHP, you can use the HTML input element with the type attribute set to "file". This will allow users to browse and select files from their local system. Once the file is selected, you can use PHP to handle the file upload process and attach the file to the email before sending it.

```php
<form action="upload.php" method="post" enctype="multipart/form-data">
    <input type="file" name="attachment">
    <input type="submit" value="Attach File">
</form>
```

In the "upload.php" file, you can handle the file upload process and attach the file to the email before sending it. Remember to add appropriate validation and security measures to prevent any potential vulnerabilities.