Allow for larger attachments

[allestuetsmerweh]

Is your feature request related to a problem? Please describe.
I have written an email forwarder, which reads the incoming messages from a catch-all mailbox using php-imap. I am generally very happy with it, but when people send email with large attachments, I see errors like these:
[...] Uncaught Exception Symfony\Component\ErrorHandler\Error\OutOfMemoryError: "Error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 9282368 bytes)" at [...]/vendor/webklex/php-imap/src/Part.php line 196

Describe the solution you'd like
It would be nice, if php-imap would not load the whole Part content to memory, but instead, e.g. write the raw message to a temp file, and only reference parts by offset within that file (or similar)

Describe alternatives you've considered
None so far, but I am very open to other solutions, to the problem!

Additional context
I would like to contribute a PR, once we know how to solve the problem.

[tiamatloc]

@allestuetsmerweh PHP is running out of RAM. Increase memory_limit in your PHP.ini and it should fix it.

[kasoft]

I have a similiar problem with e-mails that provoke a memory problem. i don't know if it's possible to 'not load' attachements when you just want to load the mail content. in my case the memory limit is 128MB per Script which seems not enough for a 8 MB File in an attachement (that i do not want to load, but which is always load in memory when you loop/check the attachement). Maybe there is a way to prevent this? At the moment i use this chain

$message = $folder->query()->getMessageByUid($this->uid);
$html = $message->getHTMLBody();
if($message->hasAttachments()) {
            foreach ($message->getAttachments() as $akey => $attachment) {
                      $attributes = $attachment->getAttributes();
                      //do soemthing with filename and size, nothing with save oder content
            }
}

After loading the Body, memory usage is already high, while saving it runs over 128MB
Memory Current: 37.4 MB MB // 39213912 Bytes
Memory Peak true: 96.3 MB MB // 100937728 Bytes
Memory Peak: 82.1 MB MB // 86113200 Bytes

Another Example: for saving a 18MB File from an E-Mail you need a memory limit of 400MB
I used Version: 5.5.0

[freescout-help]

The main reason of this issue are substr() and exlode() functions which are applied to the email content. The thing is that PHP has to allocate extra memory to perform those operations:

https://github.com/Webklex/php-imap/blob/master/src/Structure.php#L135

https://github.com/Webklex/php-imap/blob/master/src/Structure.php#L108

Here is an attempt to get rid of explode() function: freescout-help-desk/freescout@be4a123
It reduced memory consumption but not so much: 2-10% less (depending on email body size).