How can I run php script with proper permissions using the cPanel uapi?

Question

I'm creating a php script that will create an email account within cpanel. I am using the uapi Email::add_pop function with the command line using shell_exec(), as I was having other issues with the liveAPI PHP class.

I did some searching and initially, it looked like this could be caused because I did not have suexec or suphp set up. However, the only thing that changed with the error is the RUID used to read 99 instead of 1001 as it does now.

I have three files. I have a simple test form located at /home/housediablo/public_html/index.php

<form action="formhandler.php" method="post">
    username: <br>
    <input type="text" name="username"><br>
    Password: <br>
    <input type="text" name="password"><br>
    <input type="submit" value="Submit">
</form>

I have the form handler located at /home/housediablo/public_html/formhandler.php

<?php
include "/usr/local/cpanel/base/frontend/paper_lantern/createmail.live.php";
?>

And I have the actual script that is using shell_exec() located at /usr/local/cpanel/base/frontend/paper_lantern/createmail.live.php

<?php
$username = $_POST['username'];
$password = $_POST['password'];
$domain = 'housediablo.com';
$cmdString =  'uapi --user=housediablo Email add_pop email=' . $username . ' password=' . 
$password . ' quota=0 domain=' . $domain . ' skip_update_db=1';
echo ($cmdString);
$output = shell_exec($cmdString . ' 2>&1');
echo "</ br><pre>$output</pre>";
?>

The expected result is an email being created in cpanel, but I am just getting this output:

[2019-07-25 23:00:22 +0000] die [uapi] setuids failed: Attempting to setuid as a normal user with RUID 1001 at /usr/local/cpanel/Cpanel/AccessIds/SetUids.pm line 89. Cpanel::AccessIds::SetUids::_log_and_die_if_not_root() called at /usr/local/cpanel/Cpanel/AccessIds/SetUids.pm line 66 Cpanel::AccessIds::SetUids::setuids("housediablo") called at bin/apitool.pl line 109 bin::apitool::run(CPANEL_HIDDEN, CPANEL_HIDDEN, CPANEL_HIDDEN, CPANEL_HIDDEN, CPANEL_HIDDEN, CPANEL_HIDDEN, CPANEL_HIDDEN, CPANEL_HIDDEN) called at bin/apitool.pl line 38 exit level [die] [pid=19664] (setuids failed: Attempting to setuid as a normal user with RUID 1001)

I can run the echoed $cmdString as root from the terminal and it will work:

uapi --user=housediablo Email add_pop email=test password=Str0ngT3stPw! quota=0 domain=housediablo.com skip_update_db=1

I can also run the php script as root from the terminal with hardcoded dummy variables in place of the variables from POST and it will also work.