Fatal error in the array helper

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I'm getting a fatal error when using the array helper's "flatten" method.

Here's my code:
$arr  = Loader::helper('array');
$data = $this->post();
$data = $arr->flatten($data);

The error is:
Fatal error: Call to undefined function array_flat() in /home/*****/******/updates/concrete5.6.3.2/concrete/core/helpers/array.php on line 93

https://github.com/concrete5/concrete5/blob/master/web/concrete/core...

I thought the issue might have been that I'm using a lesser version of PHP but I cannot find any reference to array_flat() anywhere.