PHP post_max_size overrides upload_max_filesize
upload_max_filesize is the limit of any single file.post_max_size is the limit of the entire body of the request, which could include multiple files.
Given post_max_size = 20M and upload_max_filesize = 6M you could upload up to 3 files of 6M each. If instead post_max_size = 6M and upload_max_filesize = 20M then you could only upload one 6M file before hitting post_max_size. It doesn't help to have upload_max_size > post_max_size.
It's not obvious how to recognize going over post_max_size. $_POST and $_FILES will be empty, but $_SERVER['CONTENT_LENGTH'] will be > 0. If the client just didn't upload any post variables or files, then $_SERVER['CONTENT_LENGTH'] will be 0.
By POST file uploads are done (commonly, there are also other methods). Look into the method attribute of the form which contains the file-upload field ;)
The lowest limit of any related setting supersedes a higher setting:
See Handling file uploads: Common Pitfals which explains this in detail and how to calculate the values.
The normal method to send a file upload is POST, thus also post_max_size should be 16 Mb or more.
Incidentally, also memory_limit plays a role. It should be bigger than 16Mb, but since the default value is 128Mb, you won't see this problem. Example php.ini configuration:
post_max_size = 16Mupload_max_filesize = 16Mmemory_limit = 128MChange these value in php.ini if you've access to it, otherwise you can try to change them in an .htaccess file.
php_value upload_max_filesize 16Mphp_value post_max_size 16M This will work only if the AllowOverride settings permit it. Otherwise, you've to ask to your hosting company.