Relationship between php’s memory_limit, upload_max_filesize and post_max_filesize

Question

Bottom-line:
Do I need to be concerned about setting post_max_filesize >> memory_limit?

Details:
This answer suggests that uploaded files do not need to fit within php’s memory_limit. The php docs suggest that the entire post should fit within php’s memory limit.

I find the docs surprising and I’m hoping someone can elaborate. For example take the following php configs:

; config A
memory_limit = 50M
upload_max_filesize = 100M
post_max_filesize = 1000M
max_file_uploads = 10    

and

; config B
memory_limit = 50M
upload_max_filesize = 10M
post_max_filesize = 1000M
max_file_uploads = 100    

With these configurations I’d expect to be able to:

• upload 10x100mb files to server A,
• and 100x10mb files to server B.

I would also expect that:

• Working with any one of the 10 files uploaded to server A is a problem (100Ms of file in a 50M bag…).
• Working with any 1 of the 100 files uploaded to server B is okay (10 < 50).

While experimenting with less round but equivalently related numbers, I’ve found these expectations hold true.

This experience would lead me to say that "generally the memory_limit should be larger than the upload_max_filesize"; instead, the php docs say:

generally speaking, memory_limit should be larger than post_max_size.

Why and what happens if it isn't?

When my php code is executed I see no evidence that all of the posted files are in memory. It seems to me that all I’ve got is a $_FILES array of paths to files found exclusively on disk. Is php holding the whole post in memory at some point prior to my ability to introspect the environment? Do I need to be concerned about setting post_max_filesize >> memory_limit?

Aside:
Violating the manual's rule does not result in a grossly broken server (w/ php5.3 apache2.2 debian 6).

Answer 1

PHP will accept uploaded files that are individually smaller than upload_max_filesize and together take less than post_max_size bytes. The PHP documentation is wrong with regard to memory_limit which does not need to hold the file contents posted.

The following configuration works with both Apache2 module and CGI, and accepts files smaller than 1G.

 upload_max_filesize = 1G
 post_max_size = 1G
 memory_limit = 32M