httpd.conf:

ExtFilterDefine MyInFilter mode=input cmd="/path/to/filters/request_filter.php"
<Location "/mywebapplication-url">
    setInputFilter MyInFilter
    ExtFilterOptions LogStderr
    LimitRequestBody 0
</Location>


My filter code:

#!/usr/bin/php
<?php

     function processFilter() {

         $stdin = file_get_contents('php://stdin');
         if (empty($stdin)) {
            error_log("empty request");
            return $stdin;
         }

         // do filter work here, do nothing for test

         return $stdin;
    }

    error_log("Filter starts");

    $before = microtime(true);
    print(processFilter());
    $after = microtime(true);

    error_log("Filter needs: " . ($after-$before) ." s");
?>

If I post a request with data larger than 8192 bytes, my filter won't be called and the request dies with an error:

[Tue Sep 06 09:58:09.470309 2016] [ext_filter:trace1] [pid 15343] mod_ext_filter.c(629): [client 192.168.50.26:46497] filtering `/an-URL' of type `(unspecified)' through `/path/to/filters/request_filter.php', cfg ExtFilterOptions LogStderr !PreserveContentLength ExtFilterInType application/x-www-form-urlencoded ExtFilterOuttype (unchanged), referer: https://another-URL

[Tue Sep 06 09:58:09.470399 2016] [ext_filter:trace6] [pid 15343] mod_ext_filter.c(807): (11)Resource temporarily unavailable: [client 192.168.50.26:46497] AH01466: apr_file_read(child output), len 18446744073709551615, referer: https://another-URL

[Tue Sep 06 09:58:09.470441 2016] [:error] [pid 15343] Sending error response: The request contained fewer content data than specified by the content-length header

I have recompiled the mod_ext_filter.c with a lot of log output and I found a discrepancy in ef_unified_filter(), where the apr_bucket_read() gets only 8192 bytes, but the ef_input_filter() reads just before i.e. 9827 bytes from the current brigade. 
There is only one bucket within the brigade and the for-loop in ef_unified_filter() runs therefore only once. The result is the different content length from the error message, but at the moment I cannot find a solution for that.

It is not a PHP-specific problem, I have tested it with a Bash-script filter too. Also I have tested w/o LimitRequestBody.