Recently we faced an issue of form not submitting with all form fields. Let me describe the issue in details.
In CiviCRM, when creating a new Contact or updating an old one, selected tags were not saving. This was working before but suddenly this issue arose. After thorough investigation we discovered that not all fields of the form was posted or in other words the PHP $_POST variable have not listed all fields. In our Windows machine we got all the fields but in Linux box the issue existed. First we thought the issue might have caused due to the PHP directive “post_max_size”, though it was set to 24M. We tried with updating it to 128M. But the issue was still there.
Then we searched Google and got a clue from http://akrabat.com/php/missing-fields-in-_post/. Our server was using PHP 5.3.22. So, we looked for the “max_input_vars” directive which was new since 5.3.9, it was set to 1000 by default. We increased the number to 2000 and the issue was resolved.
Due to the number of groups in the form, the field number increased to more than 1000. So, PHP was truncating the POST data and at most 1000 fields were returned. That’s why Tags fields were not listed in the $_POST variable.