Rock, Papper, Sizor game!

<?php

// Computer Vs User Game [Rock, Sizor, Papper]!

// Check the Argumets Number!
if(count($argv) >= 3)
    die("Invalid Inputs!\n[!] The Script Excepts 1 parameter.\n");

$AllowedCharacters = ['r', 'p', 's'];
$UserInput = $argv[1];

// Check the availability of the inputs.
if(!in_array($UserInput, $AllowedCharacters))
    die("invalid Input!\n[!] {r} -> Rock, {p} -> Papper, {S} -> Sizor.\n");

// Computer Random Output!
// return: @string
function CompyterPlay() {
    global $AllowedCharacters, $ArrayKey;
    $ArrayKey = mt_rand(0, 2);
    return $AllowedCharacters[$ArrayKey];
}

// Check the winner.
if(     $UserInput == 'r' && CompyterPlay() == 's'
    ||  $UserInput == 's' && CompyterPlay() == 'p'
    ||  $UserInput == 'p' && CompyterPlay() == 'r') {
    echo 'User Wins!';
} else {
    echo 'Computer Wins!';
}