$_POST key/value not set if length > 1000000 characters [duplicate]

Here is some data that I am POSTing to a PHP script.

$Data = [ "Something" => "1", "SomethingElse" => "2", "More" => "3", "Exactly1000000" => substr(file_get_contents("Data2MB.txt"), 0, 1000000), "Exactly1000001" => substr(file_get_contents("Data2MB.txt"), 0, 1000001), "About2MB" => file_get_contents("Data2MB.txt"), ]; 

This has been done using cURL, Guzzle (not using cURL) and via XMLHttpRequest. Both produce identical results.

Posted to:

<?php print_r($_POST); ?> 

Output:

Array ( [Something] => 1 [SomethingElse] => 2 [More] => 3 [Exactly1000000] => Z0vp2++85+y9wjFL... (very long random base64) ) 

Any string value > 1000000 characters literally disappears BUT is present if the target script shows file_get_contents("php://input")

I have the following PHP parameters set:

max_execution_time 500 max_file_uploads 20 max_input_nesting_level 64 max_input_time 500 max_input_vars 10000 memory_limit 512M post_max_size 240M 

Can anyone advise me on what is going on?

Edit: No, it's not a duplicate question. I know about the upper limits, which can be huge. My example is only sending megabytes and the PHP settings have been altered (above) to create more than enough headroom. For some reason, the keys/values are simply not being seen on my server.